Csdrhrt

When you first turn on your home router, how does it acquire a public IP address ? Is it a statically assigned IP address given to you by your ISP provider ? Or does the router act as a DHCP client in obtaining a dynamic address. If the router acts as a DHCP client, which server does it request to for an IP address ? And does it have to do a DHCP request every time you turn on the router ?

When you first turn on your home router, how does it acquire a public IP address ?

In most cases any new SOHO router is set to obtain external IP dynamically (DHCP). Any other setting needs some hand-set values for proper connection.

which server does it request to for an IP address ?

Router sends its DHCP request like a broadcast. So the provider's DHCP accepts it (if it exists and if current client connection must use DHCP).

does it have to do a DHCP request every time you turn on the router ?

Yes.

Moreover, when router's address is set from DHCP, it has a property of its max. possible age (leased time), and when this time expired (in real - when near 80% of leased time expires) the router sends DHCP request again to renew the lease and reset leased time (but DHCP server in some cases may not renew, but alter leased IP address).

It actually depends a bit on your modem, but broadly that's how it works. There might be different protocols and setup until your modem, - with various protocols like PPPoe and such

here's an example of various ones my old router supports. (I have it disabled since I use it as an AP). Many of these run DHCP over the other protocol.

These may optionally use DHCP or static addressing for IPv4 depending on your ISP, and the lease times depend entirely on your ISP.

I run my own linux router, on a perfectly vanilla ONT - and it uses the same basic settings as it would as it was a client - automatic DHCP.

Looking at DHCP client logs (not my actual IPs - swapped them out for RFC 5737 IPs, and swapped out some other things for bogus values)

Nov 13 09:20:44 heckate_router dhclient[1231]: Internet Systems Consortium DHCP Client 4.3.5

Nov 13 09:20:44 heckate_router dhclient[1231]: Copyright 2004-2016 Internet Systems Consortium.

Nov 13 09:20:44 heckate_router dhclient[1231]: All rights reserved.

Nov 13 09:20:44 heckate_router dhclient[1231]: For info, please visit https://www.isc.org/software/dhcp/

Nov 13 09:20:44 heckate_router dhclient[1231]:

Nov 13 09:20:44 heckate_router dhclient[1231]: Listening on LPF/enp1s0/FF:FF:FF:FF:FF:FF

Nov 13 09:20:44 heckate_router dhclient[1231]: Sending on   LPF/enp1s0/FF:FF:FF:FF:FF:FF

Nov 13 09:20:44 heckate_router dhclient[1231]: Sending on   Socket/fallback

Nov 13 09:20:44 heckate_router dhclient[1231]: DHCPDISCOVER on enp1s0 to 255.255.255.255 port 67 interval 3 (xid=0xDEADBEEF)

Nov 13 09:20:47 heckate_router dhclient[1231]: DHCPDISCOVER on enp1s0 to 255.255.255.255 port 67 interval 8 (xid=0xDEADBEEF)

Nov 13 09:20:55 heckate_router dhclient[1231]: DHCPDISCOVER on enp1s0 to 255.255.255.255 port 67 interval 17 (xid=0xDEADBEEF)

Nov 13 09:20:55 heckate_router dhclient[1231]: DHCPREQUEST of 192.0.2.197 on enp1s0 to 255.255.255.255 port 67 (xid=0xDEADBEEF)

Nov 13 09:20:55 heckate_router dhclient[1231]: DHCPOFFER of 192.0.2.197 from 203.0.113.3

Nov 13 09:20:55 heckate_router dhclient[1231]: DHCPACK of 192.0.2.197 from 203.0.113.3

Nov 13 09:20:55 heckate_router dhclient[1231]: bound to 192.0.2.197 -- renewal in 74135 seconds.

Nov 14 05:56:30 heckate_router dhclient[1794]: DHCPREQUEST of 192.0.2.197 on enp1s0 to 198.51.100.201 port 67 (xid=0xDEADBEEF)

Nov 14 05:56:30 heckate_router dhclient[1794]: DHCPACK of 192.0.2.197 from 172.17.0.201

Nov 14 05:56:30 heckate_router dhclient[1794]: bound to 192.0.2.197 -- renewal in 88638 seconds.

This is almost the same way it would work inside a lan, and the IP address lease is refreshed periodically. In fact, on the "internet" facing end, once the underlying protocol is negotiated (in my case by the ONT, or in other cases your modem), it is indistinguishable from a client machine in your lan

In the context of the service you contracted from your ISP and their network :

• Your ISP issue router gets a static IP address

or

• Your ISP issue router gets a dynamic IP address

The specific protocol that handles the IP assignment can vary depending on how up-to-date your ISP is in terms of equipment.

Assuming your ISP is using DHCP to address your ISP issue router your router is indeed a client to it and it get's it's address from a server within your provider's network.
When you first connect your router it does send a broadcast to the network requesting an address. When it get's one the type of address comes into play. If you have a static IP address this address will not change.

But more commonly a dynamic address is contracted and it has a lease time, that being how long it lasts until requiring renewal.
If you shutdown your router, and turn it back it won't request a new address because it already has one, assuming the lease is still valid ( If the router has the address in memory or the server keeps track of who is what address is another issue). Usually when the lease is halfway through it's time, a new IP is requested.