Why do IPv6 unique local addresses have to have a /48 prefix?












5















According to RFC 4193, unique local addresses will always have a prefix of FD00::/8.. but according to Wikipedia:




The block fd00::/8 is defined for /48 prefixes, formed by setting the forty least-significant bits of the prefix to a randomly generated bit string.




Is this enforced, and if so why? What stops me from having a prefix of /32 or /16 etc?










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  • 2





    Remember the the "U" in ULA stands for "unique."

    – Ron Maupin
    Mar 24 at 16:03
















5















According to RFC 4193, unique local addresses will always have a prefix of FD00::/8.. but according to Wikipedia:




The block fd00::/8 is defined for /48 prefixes, formed by setting the forty least-significant bits of the prefix to a randomly generated bit string.




Is this enforced, and if so why? What stops me from having a prefix of /32 or /16 etc?










share|improve this question









New contributor




Sam P is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
















  • 2





    Remember the the "U" in ULA stands for "unique."

    – Ron Maupin
    Mar 24 at 16:03














5












5








5








According to RFC 4193, unique local addresses will always have a prefix of FD00::/8.. but according to Wikipedia:




The block fd00::/8 is defined for /48 prefixes, formed by setting the forty least-significant bits of the prefix to a randomly generated bit string.




Is this enforced, and if so why? What stops me from having a prefix of /32 or /16 etc?










share|improve this question









New contributor




Sam P is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












According to RFC 4193, unique local addresses will always have a prefix of FD00::/8.. but according to Wikipedia:




The block fd00::/8 is defined for /48 prefixes, formed by setting the forty least-significant bits of the prefix to a randomly generated bit string.




Is this enforced, and if so why? What stops me from having a prefix of /32 or /16 etc?







ip network ipv6 protocol-theory networking






share|improve this question









New contributor




Sam P is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|improve this question









New contributor




Sam P is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|improve this question




share|improve this question








edited Mar 24 at 17:52









Ron Maupin

67.9k1369126




67.9k1369126






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asked Mar 24 at 7:03









Sam PSam P

1283




1283




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Sam P is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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New contributor





Sam P is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






Sam P is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.








  • 2





    Remember the the "U" in ULA stands for "unique."

    – Ron Maupin
    Mar 24 at 16:03














  • 2





    Remember the the "U" in ULA stands for "unique."

    – Ron Maupin
    Mar 24 at 16:03








2




2





Remember the the "U" in ULA stands for "unique."

– Ron Maupin
Mar 24 at 16:03





Remember the the "U" in ULA stands for "unique."

– Ron Maupin
Mar 24 at 16:03










2 Answers
2






active

oldest

votes


















11














The requirement exists to prevent collisions. This is a bit more important than most people recognize.



Even if you have systems which currently don't communicate with other systems over the internet you still need your addresses to be globally unique. You may now or in the future need to add a host which can communicate both with your internal network and with the internet. And for communication with that host to work, the IP addresses with which it communicate will need to be unique.



If two internal networks exist with the same local range there is the possibility a host will eventually need to communicate with both and at that point you would have to renumber one of the networks. This kind of communication is likely to be needed if you are using a VPN connection and both client and server are on networks which make use of RFC 4193 address space.



Another way you could end up needing to communicate with other internal networks in the future is in case your company merges with another company which also uses internal networks.



40 random bits is enough to guarantee that a host which needs to communicate with multiple internal networks can expect to reach approximately one million different networks before seeing the first collision.



The 40 random bits requirement is not enforced in any way, but if you don't follow it you are setting yourself up for problems in the future when a collision happens.






share|improve this answer



















  • 1





    I'd be careful with the words "guarantee" and "expect"; there is a chance you will find a collision after one attempt. Not a large chance, sure, but..!

    – Lightness Races in Orbit
    Mar 24 at 20:24






  • 4





    @LightnessRacesinOrbit The word expect is being used according to the mathematical definition of expected value.

    – kasperd
    Mar 24 at 20:28






  • 1





    Then you should add an explanation or disclaimer to that effect, IMO, since this is not MathOverflow :)

    – Lightness Races in Orbit
    Mar 24 at 20:31






  • 1





    @LightnessRacesinOrbit I actually do expect network engineers to know some probability theory. This would be taught in an introductory course. Besides the risk of seeing a collision much sooner is much smaller than so many of the other things people take for granted.

    – kasperd
    Mar 24 at 20:46






  • 1





    Even so, you can't "guarantee" an "expectation", and I think your math is wrong anyway. With 40 bits of entropy the birthday paradox means that you need of the order of 1M (2^20) networks for there to be a 50% chance of any collision existing for any arbitrary pair of networks amongst all possible pairs. The chance of a collision existing for your network is 2^40 (assuming that your own ULA prefix was truly randomly selected).

    – Alnitak
    Mar 24 at 21:16





















9














When companies merge or set up an extranet to communicate, it has proven difficult with IPv4 Private addressing because the companies often use the same or overlapping address space, and that requires the ugly hack of NAT to get around, and that can cause problems and break many protocols.



This was identified as a problem when IPv6 ULA was being developed, and the goal was to allow companies to have non-Internet address space, but to have a very high probability that the space used was unique. This is to try to prevent the problem of merging or communication between companies using non-Internet addressing. IPv6 doesn't have NAT, and the goal of IPv6 is to restore the IP end-to-end connectivity that was lost when NAT became necessary due to the limited number of IPv4 addresses.



The first half of the IPv6 ULA space (fc00::/8) is reserved for assignment by a (yet to be named) global authority, while the second half of the IPv6 ULA space (fd00::/8) was set up so that companies could assign their own addressing with a high probability of uniqueness.






According to RFC 4193, unique local addresses will always have a
prefix of FD00::/8




That is simply incorrect. That RFC defines the ULA space as fc00::/7, but there are two parts to the space that are defined by the eighth bit ("L" bit).



From the RFC:




3.1. Format



The Local IPv6 addresses are created using a pseudo-randomly allocated
global ID. They have the following format:



| 7 bits |1|  40 bits   |  16 bits  |          64 bits           |
+--------+-+------------+-----------+----------------------------+
| Prefix |L| Global ID | Subnet ID | Interface ID |
+--------+-+------------+-----------+----------------------------+



This divides the ULA space into two /8 spaces: fc00::/8 for globally assigned addressing, and fd00::/8 for locally assigned addressing. Notice the format in the RFC requires "a pseudo-randomly allocated global ID." This is further explained:




3.2. Global ID



The allocation of Global IDs is pseudo-random [RANDOM]. They MUST NOT
be assigned sequentially or with well-known numbers. This is to ensure
that there is not any relationship between allocations and to help
clarify that these prefixes are not intended to be routed globally.
Specifically, these prefixes are not designed to aggregate.



This document defines a specific local method to allocate Global IDs,
indicated by setting the L bit to 1. Another method, indicated by
clearing the L bit, may be defined later. Apart from the allocation
method, all Local IPv6 addresses behave and are treated identically.



The local assignments are self-generated and do not need any central
coordination or assignment, but have an extremely high probability of
being unique.




As you can see, the premise of your question that the RFC says that ULA addresses will always have a prefix of fd00::/8 is incorrect.




Is this enforced, and if so why? What stops me from having a prefix of
/32 or /16 etc?




There is no actual enforcement, the way there would be if you were trying to use the addressing on the public Internet. Your company could simply use any addressing in that space, in whatever blocks it wants. What your business does for addressing on its own network is completely up to it, but it could prove foolish and expensive in the long run to not follow the standards.



For example, I know of some companies that used "dark" IPv4 address space within their networks, but then the dark address space started to be used on the public Internet, and the companies were unable to connect with customers or vendors using addressing in that address space, and it took some ugly solutions to get around that in the short term, until all the internal networks using that address space were readdressed. It took a few years and a lot of money to fix the problems.





RFC 4193, Unique Local IPv6 Unicast Addresses is the definition of IPv6 ULA, and you should refer to it for the details:




1. Introduction



This document defines an IPv6 unicast address format that is globally
unique and is intended for local communications [IPV6]. These
addresses are called Unique Local IPv6 Unicast Addresses and are
abbreviated in this document as Local IPv6 addresses. They are not
expected to be routable on the global Internet. They are routable
inside of a more limited area such as a site. They may also be routed
between a limited set of sites.



Local IPv6 unicast addresses have the following characteristics:




  • Globally unique prefix (with high probability of uniqueness).


  • Well-known prefix to allow for easy filtering at site
    boundaries.


  • Allow sites to be combined or privately interconnected without creating any address conflicts or requiring renumbering of
    interfaces that use these prefixes.


  • Internet Service Provider independent and can be used for
    communications inside of a site without having any permanent or
    intermittent Internet connectivity.


  • If accidentally leaked outside of a site via routing or DNS,
    there is no conflict with any other addresses.


  • In practice, applications may treat these addresses like global
    scoped addresses.



This document defines the format of Local IPv6 addresses, how to
allocate them, and usage considerations including routing, site border
routers, DNS, application support, VPN usage, and guidelines for how
to use for local communication inside a site.







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    2 Answers
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    2 Answers
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    11














    The requirement exists to prevent collisions. This is a bit more important than most people recognize.



    Even if you have systems which currently don't communicate with other systems over the internet you still need your addresses to be globally unique. You may now or in the future need to add a host which can communicate both with your internal network and with the internet. And for communication with that host to work, the IP addresses with which it communicate will need to be unique.



    If two internal networks exist with the same local range there is the possibility a host will eventually need to communicate with both and at that point you would have to renumber one of the networks. This kind of communication is likely to be needed if you are using a VPN connection and both client and server are on networks which make use of RFC 4193 address space.



    Another way you could end up needing to communicate with other internal networks in the future is in case your company merges with another company which also uses internal networks.



    40 random bits is enough to guarantee that a host which needs to communicate with multiple internal networks can expect to reach approximately one million different networks before seeing the first collision.



    The 40 random bits requirement is not enforced in any way, but if you don't follow it you are setting yourself up for problems in the future when a collision happens.






    share|improve this answer



















    • 1





      I'd be careful with the words "guarantee" and "expect"; there is a chance you will find a collision after one attempt. Not a large chance, sure, but..!

      – Lightness Races in Orbit
      Mar 24 at 20:24






    • 4





      @LightnessRacesinOrbit The word expect is being used according to the mathematical definition of expected value.

      – kasperd
      Mar 24 at 20:28






    • 1





      Then you should add an explanation or disclaimer to that effect, IMO, since this is not MathOverflow :)

      – Lightness Races in Orbit
      Mar 24 at 20:31






    • 1





      @LightnessRacesinOrbit I actually do expect network engineers to know some probability theory. This would be taught in an introductory course. Besides the risk of seeing a collision much sooner is much smaller than so many of the other things people take for granted.

      – kasperd
      Mar 24 at 20:46






    • 1





      Even so, you can't "guarantee" an "expectation", and I think your math is wrong anyway. With 40 bits of entropy the birthday paradox means that you need of the order of 1M (2^20) networks for there to be a 50% chance of any collision existing for any arbitrary pair of networks amongst all possible pairs. The chance of a collision existing for your network is 2^40 (assuming that your own ULA prefix was truly randomly selected).

      – Alnitak
      Mar 24 at 21:16


















    11














    The requirement exists to prevent collisions. This is a bit more important than most people recognize.



    Even if you have systems which currently don't communicate with other systems over the internet you still need your addresses to be globally unique. You may now or in the future need to add a host which can communicate both with your internal network and with the internet. And for communication with that host to work, the IP addresses with which it communicate will need to be unique.



    If two internal networks exist with the same local range there is the possibility a host will eventually need to communicate with both and at that point you would have to renumber one of the networks. This kind of communication is likely to be needed if you are using a VPN connection and both client and server are on networks which make use of RFC 4193 address space.



    Another way you could end up needing to communicate with other internal networks in the future is in case your company merges with another company which also uses internal networks.



    40 random bits is enough to guarantee that a host which needs to communicate with multiple internal networks can expect to reach approximately one million different networks before seeing the first collision.



    The 40 random bits requirement is not enforced in any way, but if you don't follow it you are setting yourself up for problems in the future when a collision happens.






    share|improve this answer



















    • 1





      I'd be careful with the words "guarantee" and "expect"; there is a chance you will find a collision after one attempt. Not a large chance, sure, but..!

      – Lightness Races in Orbit
      Mar 24 at 20:24






    • 4





      @LightnessRacesinOrbit The word expect is being used according to the mathematical definition of expected value.

      – kasperd
      Mar 24 at 20:28






    • 1





      Then you should add an explanation or disclaimer to that effect, IMO, since this is not MathOverflow :)

      – Lightness Races in Orbit
      Mar 24 at 20:31






    • 1





      @LightnessRacesinOrbit I actually do expect network engineers to know some probability theory. This would be taught in an introductory course. Besides the risk of seeing a collision much sooner is much smaller than so many of the other things people take for granted.

      – kasperd
      Mar 24 at 20:46






    • 1





      Even so, you can't "guarantee" an "expectation", and I think your math is wrong anyway. With 40 bits of entropy the birthday paradox means that you need of the order of 1M (2^20) networks for there to be a 50% chance of any collision existing for any arbitrary pair of networks amongst all possible pairs. The chance of a collision existing for your network is 2^40 (assuming that your own ULA prefix was truly randomly selected).

      – Alnitak
      Mar 24 at 21:16
















    11












    11








    11







    The requirement exists to prevent collisions. This is a bit more important than most people recognize.



    Even if you have systems which currently don't communicate with other systems over the internet you still need your addresses to be globally unique. You may now or in the future need to add a host which can communicate both with your internal network and with the internet. And for communication with that host to work, the IP addresses with which it communicate will need to be unique.



    If two internal networks exist with the same local range there is the possibility a host will eventually need to communicate with both and at that point you would have to renumber one of the networks. This kind of communication is likely to be needed if you are using a VPN connection and both client and server are on networks which make use of RFC 4193 address space.



    Another way you could end up needing to communicate with other internal networks in the future is in case your company merges with another company which also uses internal networks.



    40 random bits is enough to guarantee that a host which needs to communicate with multiple internal networks can expect to reach approximately one million different networks before seeing the first collision.



    The 40 random bits requirement is not enforced in any way, but if you don't follow it you are setting yourself up for problems in the future when a collision happens.






    share|improve this answer













    The requirement exists to prevent collisions. This is a bit more important than most people recognize.



    Even if you have systems which currently don't communicate with other systems over the internet you still need your addresses to be globally unique. You may now or in the future need to add a host which can communicate both with your internal network and with the internet. And for communication with that host to work, the IP addresses with which it communicate will need to be unique.



    If two internal networks exist with the same local range there is the possibility a host will eventually need to communicate with both and at that point you would have to renumber one of the networks. This kind of communication is likely to be needed if you are using a VPN connection and both client and server are on networks which make use of RFC 4193 address space.



    Another way you could end up needing to communicate with other internal networks in the future is in case your company merges with another company which also uses internal networks.



    40 random bits is enough to guarantee that a host which needs to communicate with multiple internal networks can expect to reach approximately one million different networks before seeing the first collision.



    The 40 random bits requirement is not enforced in any way, but if you don't follow it you are setting yourself up for problems in the future when a collision happens.







    share|improve this answer












    share|improve this answer



    share|improve this answer










    answered Mar 24 at 13:29









    kasperdkasperd

    7141613




    7141613








    • 1





      I'd be careful with the words "guarantee" and "expect"; there is a chance you will find a collision after one attempt. Not a large chance, sure, but..!

      – Lightness Races in Orbit
      Mar 24 at 20:24






    • 4





      @LightnessRacesinOrbit The word expect is being used according to the mathematical definition of expected value.

      – kasperd
      Mar 24 at 20:28






    • 1





      Then you should add an explanation or disclaimer to that effect, IMO, since this is not MathOverflow :)

      – Lightness Races in Orbit
      Mar 24 at 20:31






    • 1





      @LightnessRacesinOrbit I actually do expect network engineers to know some probability theory. This would be taught in an introductory course. Besides the risk of seeing a collision much sooner is much smaller than so many of the other things people take for granted.

      – kasperd
      Mar 24 at 20:46






    • 1





      Even so, you can't "guarantee" an "expectation", and I think your math is wrong anyway. With 40 bits of entropy the birthday paradox means that you need of the order of 1M (2^20) networks for there to be a 50% chance of any collision existing for any arbitrary pair of networks amongst all possible pairs. The chance of a collision existing for your network is 2^40 (assuming that your own ULA prefix was truly randomly selected).

      – Alnitak
      Mar 24 at 21:16
















    • 1





      I'd be careful with the words "guarantee" and "expect"; there is a chance you will find a collision after one attempt. Not a large chance, sure, but..!

      – Lightness Races in Orbit
      Mar 24 at 20:24






    • 4





      @LightnessRacesinOrbit The word expect is being used according to the mathematical definition of expected value.

      – kasperd
      Mar 24 at 20:28






    • 1





      Then you should add an explanation or disclaimer to that effect, IMO, since this is not MathOverflow :)

      – Lightness Races in Orbit
      Mar 24 at 20:31






    • 1





      @LightnessRacesinOrbit I actually do expect network engineers to know some probability theory. This would be taught in an introductory course. Besides the risk of seeing a collision much sooner is much smaller than so many of the other things people take for granted.

      – kasperd
      Mar 24 at 20:46






    • 1





      Even so, you can't "guarantee" an "expectation", and I think your math is wrong anyway. With 40 bits of entropy the birthday paradox means that you need of the order of 1M (2^20) networks for there to be a 50% chance of any collision existing for any arbitrary pair of networks amongst all possible pairs. The chance of a collision existing for your network is 2^40 (assuming that your own ULA prefix was truly randomly selected).

      – Alnitak
      Mar 24 at 21:16










    1




    1





    I'd be careful with the words "guarantee" and "expect"; there is a chance you will find a collision after one attempt. Not a large chance, sure, but..!

    – Lightness Races in Orbit
    Mar 24 at 20:24





    I'd be careful with the words "guarantee" and "expect"; there is a chance you will find a collision after one attempt. Not a large chance, sure, but..!

    – Lightness Races in Orbit
    Mar 24 at 20:24




    4




    4





    @LightnessRacesinOrbit The word expect is being used according to the mathematical definition of expected value.

    – kasperd
    Mar 24 at 20:28





    @LightnessRacesinOrbit The word expect is being used according to the mathematical definition of expected value.

    – kasperd
    Mar 24 at 20:28




    1




    1





    Then you should add an explanation or disclaimer to that effect, IMO, since this is not MathOverflow :)

    – Lightness Races in Orbit
    Mar 24 at 20:31





    Then you should add an explanation or disclaimer to that effect, IMO, since this is not MathOverflow :)

    – Lightness Races in Orbit
    Mar 24 at 20:31




    1




    1





    @LightnessRacesinOrbit I actually do expect network engineers to know some probability theory. This would be taught in an introductory course. Besides the risk of seeing a collision much sooner is much smaller than so many of the other things people take for granted.

    – kasperd
    Mar 24 at 20:46





    @LightnessRacesinOrbit I actually do expect network engineers to know some probability theory. This would be taught in an introductory course. Besides the risk of seeing a collision much sooner is much smaller than so many of the other things people take for granted.

    – kasperd
    Mar 24 at 20:46




    1




    1





    Even so, you can't "guarantee" an "expectation", and I think your math is wrong anyway. With 40 bits of entropy the birthday paradox means that you need of the order of 1M (2^20) networks for there to be a 50% chance of any collision existing for any arbitrary pair of networks amongst all possible pairs. The chance of a collision existing for your network is 2^40 (assuming that your own ULA prefix was truly randomly selected).

    – Alnitak
    Mar 24 at 21:16







    Even so, you can't "guarantee" an "expectation", and I think your math is wrong anyway. With 40 bits of entropy the birthday paradox means that you need of the order of 1M (2^20) networks for there to be a 50% chance of any collision existing for any arbitrary pair of networks amongst all possible pairs. The chance of a collision existing for your network is 2^40 (assuming that your own ULA prefix was truly randomly selected).

    – Alnitak
    Mar 24 at 21:16













    9














    When companies merge or set up an extranet to communicate, it has proven difficult with IPv4 Private addressing because the companies often use the same or overlapping address space, and that requires the ugly hack of NAT to get around, and that can cause problems and break many protocols.



    This was identified as a problem when IPv6 ULA was being developed, and the goal was to allow companies to have non-Internet address space, but to have a very high probability that the space used was unique. This is to try to prevent the problem of merging or communication between companies using non-Internet addressing. IPv6 doesn't have NAT, and the goal of IPv6 is to restore the IP end-to-end connectivity that was lost when NAT became necessary due to the limited number of IPv4 addresses.



    The first half of the IPv6 ULA space (fc00::/8) is reserved for assignment by a (yet to be named) global authority, while the second half of the IPv6 ULA space (fd00::/8) was set up so that companies could assign their own addressing with a high probability of uniqueness.






    According to RFC 4193, unique local addresses will always have a
    prefix of FD00::/8




    That is simply incorrect. That RFC defines the ULA space as fc00::/7, but there are two parts to the space that are defined by the eighth bit ("L" bit).



    From the RFC:




    3.1. Format



    The Local IPv6 addresses are created using a pseudo-randomly allocated
    global ID. They have the following format:



    | 7 bits |1|  40 bits   |  16 bits  |          64 bits           |
    +--------+-+------------+-----------+----------------------------+
    | Prefix |L| Global ID | Subnet ID | Interface ID |
    +--------+-+------------+-----------+----------------------------+



    This divides the ULA space into two /8 spaces: fc00::/8 for globally assigned addressing, and fd00::/8 for locally assigned addressing. Notice the format in the RFC requires "a pseudo-randomly allocated global ID." This is further explained:




    3.2. Global ID



    The allocation of Global IDs is pseudo-random [RANDOM]. They MUST NOT
    be assigned sequentially or with well-known numbers. This is to ensure
    that there is not any relationship between allocations and to help
    clarify that these prefixes are not intended to be routed globally.
    Specifically, these prefixes are not designed to aggregate.



    This document defines a specific local method to allocate Global IDs,
    indicated by setting the L bit to 1. Another method, indicated by
    clearing the L bit, may be defined later. Apart from the allocation
    method, all Local IPv6 addresses behave and are treated identically.



    The local assignments are self-generated and do not need any central
    coordination or assignment, but have an extremely high probability of
    being unique.




    As you can see, the premise of your question that the RFC says that ULA addresses will always have a prefix of fd00::/8 is incorrect.




    Is this enforced, and if so why? What stops me from having a prefix of
    /32 or /16 etc?




    There is no actual enforcement, the way there would be if you were trying to use the addressing on the public Internet. Your company could simply use any addressing in that space, in whatever blocks it wants. What your business does for addressing on its own network is completely up to it, but it could prove foolish and expensive in the long run to not follow the standards.



    For example, I know of some companies that used "dark" IPv4 address space within their networks, but then the dark address space started to be used on the public Internet, and the companies were unable to connect with customers or vendors using addressing in that address space, and it took some ugly solutions to get around that in the short term, until all the internal networks using that address space were readdressed. It took a few years and a lot of money to fix the problems.





    RFC 4193, Unique Local IPv6 Unicast Addresses is the definition of IPv6 ULA, and you should refer to it for the details:




    1. Introduction



    This document defines an IPv6 unicast address format that is globally
    unique and is intended for local communications [IPV6]. These
    addresses are called Unique Local IPv6 Unicast Addresses and are
    abbreviated in this document as Local IPv6 addresses. They are not
    expected to be routable on the global Internet. They are routable
    inside of a more limited area such as a site. They may also be routed
    between a limited set of sites.



    Local IPv6 unicast addresses have the following characteristics:




    • Globally unique prefix (with high probability of uniqueness).


    • Well-known prefix to allow for easy filtering at site
      boundaries.


    • Allow sites to be combined or privately interconnected without creating any address conflicts or requiring renumbering of
      interfaces that use these prefixes.


    • Internet Service Provider independent and can be used for
      communications inside of a site without having any permanent or
      intermittent Internet connectivity.


    • If accidentally leaked outside of a site via routing or DNS,
      there is no conflict with any other addresses.


    • In practice, applications may treat these addresses like global
      scoped addresses.



    This document defines the format of Local IPv6 addresses, how to
    allocate them, and usage considerations including routing, site border
    routers, DNS, application support, VPN usage, and guidelines for how
    to use for local communication inside a site.







    share|improve this answer






























      9














      When companies merge or set up an extranet to communicate, it has proven difficult with IPv4 Private addressing because the companies often use the same or overlapping address space, and that requires the ugly hack of NAT to get around, and that can cause problems and break many protocols.



      This was identified as a problem when IPv6 ULA was being developed, and the goal was to allow companies to have non-Internet address space, but to have a very high probability that the space used was unique. This is to try to prevent the problem of merging or communication between companies using non-Internet addressing. IPv6 doesn't have NAT, and the goal of IPv6 is to restore the IP end-to-end connectivity that was lost when NAT became necessary due to the limited number of IPv4 addresses.



      The first half of the IPv6 ULA space (fc00::/8) is reserved for assignment by a (yet to be named) global authority, while the second half of the IPv6 ULA space (fd00::/8) was set up so that companies could assign their own addressing with a high probability of uniqueness.






      According to RFC 4193, unique local addresses will always have a
      prefix of FD00::/8




      That is simply incorrect. That RFC defines the ULA space as fc00::/7, but there are two parts to the space that are defined by the eighth bit ("L" bit).



      From the RFC:




      3.1. Format



      The Local IPv6 addresses are created using a pseudo-randomly allocated
      global ID. They have the following format:



      | 7 bits |1|  40 bits   |  16 bits  |          64 bits           |
      +--------+-+------------+-----------+----------------------------+
      | Prefix |L| Global ID | Subnet ID | Interface ID |
      +--------+-+------------+-----------+----------------------------+



      This divides the ULA space into two /8 spaces: fc00::/8 for globally assigned addressing, and fd00::/8 for locally assigned addressing. Notice the format in the RFC requires "a pseudo-randomly allocated global ID." This is further explained:




      3.2. Global ID



      The allocation of Global IDs is pseudo-random [RANDOM]. They MUST NOT
      be assigned sequentially or with well-known numbers. This is to ensure
      that there is not any relationship between allocations and to help
      clarify that these prefixes are not intended to be routed globally.
      Specifically, these prefixes are not designed to aggregate.



      This document defines a specific local method to allocate Global IDs,
      indicated by setting the L bit to 1. Another method, indicated by
      clearing the L bit, may be defined later. Apart from the allocation
      method, all Local IPv6 addresses behave and are treated identically.



      The local assignments are self-generated and do not need any central
      coordination or assignment, but have an extremely high probability of
      being unique.




      As you can see, the premise of your question that the RFC says that ULA addresses will always have a prefix of fd00::/8 is incorrect.




      Is this enforced, and if so why? What stops me from having a prefix of
      /32 or /16 etc?




      There is no actual enforcement, the way there would be if you were trying to use the addressing on the public Internet. Your company could simply use any addressing in that space, in whatever blocks it wants. What your business does for addressing on its own network is completely up to it, but it could prove foolish and expensive in the long run to not follow the standards.



      For example, I know of some companies that used "dark" IPv4 address space within their networks, but then the dark address space started to be used on the public Internet, and the companies were unable to connect with customers or vendors using addressing in that address space, and it took some ugly solutions to get around that in the short term, until all the internal networks using that address space were readdressed. It took a few years and a lot of money to fix the problems.





      RFC 4193, Unique Local IPv6 Unicast Addresses is the definition of IPv6 ULA, and you should refer to it for the details:




      1. Introduction



      This document defines an IPv6 unicast address format that is globally
      unique and is intended for local communications [IPV6]. These
      addresses are called Unique Local IPv6 Unicast Addresses and are
      abbreviated in this document as Local IPv6 addresses. They are not
      expected to be routable on the global Internet. They are routable
      inside of a more limited area such as a site. They may also be routed
      between a limited set of sites.



      Local IPv6 unicast addresses have the following characteristics:




      • Globally unique prefix (with high probability of uniqueness).


      • Well-known prefix to allow for easy filtering at site
        boundaries.


      • Allow sites to be combined or privately interconnected without creating any address conflicts or requiring renumbering of
        interfaces that use these prefixes.


      • Internet Service Provider independent and can be used for
        communications inside of a site without having any permanent or
        intermittent Internet connectivity.


      • If accidentally leaked outside of a site via routing or DNS,
        there is no conflict with any other addresses.


      • In practice, applications may treat these addresses like global
        scoped addresses.



      This document defines the format of Local IPv6 addresses, how to
      allocate them, and usage considerations including routing, site border
      routers, DNS, application support, VPN usage, and guidelines for how
      to use for local communication inside a site.







      share|improve this answer




























        9












        9








        9







        When companies merge or set up an extranet to communicate, it has proven difficult with IPv4 Private addressing because the companies often use the same or overlapping address space, and that requires the ugly hack of NAT to get around, and that can cause problems and break many protocols.



        This was identified as a problem when IPv6 ULA was being developed, and the goal was to allow companies to have non-Internet address space, but to have a very high probability that the space used was unique. This is to try to prevent the problem of merging or communication between companies using non-Internet addressing. IPv6 doesn't have NAT, and the goal of IPv6 is to restore the IP end-to-end connectivity that was lost when NAT became necessary due to the limited number of IPv4 addresses.



        The first half of the IPv6 ULA space (fc00::/8) is reserved for assignment by a (yet to be named) global authority, while the second half of the IPv6 ULA space (fd00::/8) was set up so that companies could assign their own addressing with a high probability of uniqueness.






        According to RFC 4193, unique local addresses will always have a
        prefix of FD00::/8




        That is simply incorrect. That RFC defines the ULA space as fc00::/7, but there are two parts to the space that are defined by the eighth bit ("L" bit).



        From the RFC:




        3.1. Format



        The Local IPv6 addresses are created using a pseudo-randomly allocated
        global ID. They have the following format:



        | 7 bits |1|  40 bits   |  16 bits  |          64 bits           |
        +--------+-+------------+-----------+----------------------------+
        | Prefix |L| Global ID | Subnet ID | Interface ID |
        +--------+-+------------+-----------+----------------------------+



        This divides the ULA space into two /8 spaces: fc00::/8 for globally assigned addressing, and fd00::/8 for locally assigned addressing. Notice the format in the RFC requires "a pseudo-randomly allocated global ID." This is further explained:




        3.2. Global ID



        The allocation of Global IDs is pseudo-random [RANDOM]. They MUST NOT
        be assigned sequentially or with well-known numbers. This is to ensure
        that there is not any relationship between allocations and to help
        clarify that these prefixes are not intended to be routed globally.
        Specifically, these prefixes are not designed to aggregate.



        This document defines a specific local method to allocate Global IDs,
        indicated by setting the L bit to 1. Another method, indicated by
        clearing the L bit, may be defined later. Apart from the allocation
        method, all Local IPv6 addresses behave and are treated identically.



        The local assignments are self-generated and do not need any central
        coordination or assignment, but have an extremely high probability of
        being unique.




        As you can see, the premise of your question that the RFC says that ULA addresses will always have a prefix of fd00::/8 is incorrect.




        Is this enforced, and if so why? What stops me from having a prefix of
        /32 or /16 etc?




        There is no actual enforcement, the way there would be if you were trying to use the addressing on the public Internet. Your company could simply use any addressing in that space, in whatever blocks it wants. What your business does for addressing on its own network is completely up to it, but it could prove foolish and expensive in the long run to not follow the standards.



        For example, I know of some companies that used "dark" IPv4 address space within their networks, but then the dark address space started to be used on the public Internet, and the companies were unable to connect with customers or vendors using addressing in that address space, and it took some ugly solutions to get around that in the short term, until all the internal networks using that address space were readdressed. It took a few years and a lot of money to fix the problems.





        RFC 4193, Unique Local IPv6 Unicast Addresses is the definition of IPv6 ULA, and you should refer to it for the details:




        1. Introduction



        This document defines an IPv6 unicast address format that is globally
        unique and is intended for local communications [IPV6]. These
        addresses are called Unique Local IPv6 Unicast Addresses and are
        abbreviated in this document as Local IPv6 addresses. They are not
        expected to be routable on the global Internet. They are routable
        inside of a more limited area such as a site. They may also be routed
        between a limited set of sites.



        Local IPv6 unicast addresses have the following characteristics:




        • Globally unique prefix (with high probability of uniqueness).


        • Well-known prefix to allow for easy filtering at site
          boundaries.


        • Allow sites to be combined or privately interconnected without creating any address conflicts or requiring renumbering of
          interfaces that use these prefixes.


        • Internet Service Provider independent and can be used for
          communications inside of a site without having any permanent or
          intermittent Internet connectivity.


        • If accidentally leaked outside of a site via routing or DNS,
          there is no conflict with any other addresses.


        • In practice, applications may treat these addresses like global
          scoped addresses.



        This document defines the format of Local IPv6 addresses, how to
        allocate them, and usage considerations including routing, site border
        routers, DNS, application support, VPN usage, and guidelines for how
        to use for local communication inside a site.







        share|improve this answer















        When companies merge or set up an extranet to communicate, it has proven difficult with IPv4 Private addressing because the companies often use the same or overlapping address space, and that requires the ugly hack of NAT to get around, and that can cause problems and break many protocols.



        This was identified as a problem when IPv6 ULA was being developed, and the goal was to allow companies to have non-Internet address space, but to have a very high probability that the space used was unique. This is to try to prevent the problem of merging or communication between companies using non-Internet addressing. IPv6 doesn't have NAT, and the goal of IPv6 is to restore the IP end-to-end connectivity that was lost when NAT became necessary due to the limited number of IPv4 addresses.



        The first half of the IPv6 ULA space (fc00::/8) is reserved for assignment by a (yet to be named) global authority, while the second half of the IPv6 ULA space (fd00::/8) was set up so that companies could assign their own addressing with a high probability of uniqueness.






        According to RFC 4193, unique local addresses will always have a
        prefix of FD00::/8




        That is simply incorrect. That RFC defines the ULA space as fc00::/7, but there are two parts to the space that are defined by the eighth bit ("L" bit).



        From the RFC:




        3.1. Format



        The Local IPv6 addresses are created using a pseudo-randomly allocated
        global ID. They have the following format:



        | 7 bits |1|  40 bits   |  16 bits  |          64 bits           |
        +--------+-+------------+-----------+----------------------------+
        | Prefix |L| Global ID | Subnet ID | Interface ID |
        +--------+-+------------+-----------+----------------------------+



        This divides the ULA space into two /8 spaces: fc00::/8 for globally assigned addressing, and fd00::/8 for locally assigned addressing. Notice the format in the RFC requires "a pseudo-randomly allocated global ID." This is further explained:




        3.2. Global ID



        The allocation of Global IDs is pseudo-random [RANDOM]. They MUST NOT
        be assigned sequentially or with well-known numbers. This is to ensure
        that there is not any relationship between allocations and to help
        clarify that these prefixes are not intended to be routed globally.
        Specifically, these prefixes are not designed to aggregate.



        This document defines a specific local method to allocate Global IDs,
        indicated by setting the L bit to 1. Another method, indicated by
        clearing the L bit, may be defined later. Apart from the allocation
        method, all Local IPv6 addresses behave and are treated identically.



        The local assignments are self-generated and do not need any central
        coordination or assignment, but have an extremely high probability of
        being unique.




        As you can see, the premise of your question that the RFC says that ULA addresses will always have a prefix of fd00::/8 is incorrect.




        Is this enforced, and if so why? What stops me from having a prefix of
        /32 or /16 etc?




        There is no actual enforcement, the way there would be if you were trying to use the addressing on the public Internet. Your company could simply use any addressing in that space, in whatever blocks it wants. What your business does for addressing on its own network is completely up to it, but it could prove foolish and expensive in the long run to not follow the standards.



        For example, I know of some companies that used "dark" IPv4 address space within their networks, but then the dark address space started to be used on the public Internet, and the companies were unable to connect with customers or vendors using addressing in that address space, and it took some ugly solutions to get around that in the short term, until all the internal networks using that address space were readdressed. It took a few years and a lot of money to fix the problems.





        RFC 4193, Unique Local IPv6 Unicast Addresses is the definition of IPv6 ULA, and you should refer to it for the details:




        1. Introduction



        This document defines an IPv6 unicast address format that is globally
        unique and is intended for local communications [IPV6]. These
        addresses are called Unique Local IPv6 Unicast Addresses and are
        abbreviated in this document as Local IPv6 addresses. They are not
        expected to be routable on the global Internet. They are routable
        inside of a more limited area such as a site. They may also be routed
        between a limited set of sites.



        Local IPv6 unicast addresses have the following characteristics:




        • Globally unique prefix (with high probability of uniqueness).


        • Well-known prefix to allow for easy filtering at site
          boundaries.


        • Allow sites to be combined or privately interconnected without creating any address conflicts or requiring renumbering of
          interfaces that use these prefixes.


        • Internet Service Provider independent and can be used for
          communications inside of a site without having any permanent or
          intermittent Internet connectivity.


        • If accidentally leaked outside of a site via routing or DNS,
          there is no conflict with any other addresses.


        • In practice, applications may treat these addresses like global
          scoped addresses.



        This document defines the format of Local IPv6 addresses, how to
        allocate them, and usage considerations including routing, site border
        routers, DNS, application support, VPN usage, and guidelines for how
        to use for local communication inside a site.








        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited Mar 25 at 3:09

























        answered Mar 24 at 15:34









        Ron MaupinRon Maupin

        67.9k1369126




        67.9k1369126






















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