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Does a router need to have an IP address? if the answer is yes, then what is the router IP address used for (one reason I can think of is NAT, but not all routers are used for NAT)?

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    IP addresses are not assigned to routers (or any devices for that matter), IP addresses are assigned to network interfaces. A device with several network interfaces will require a separate addresses for each interface which fall into attached network address range. This is true for router that is (usually) attached to different network on each interface and it would be true for any end system with more than one network cards.
    – Effie
    Aug 23 at 11:23

2 Answers 2

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Theoretically no, practically yes.

There is nothing fundmamental about IP forwarding that requires the router to have an IP address, however there are several practical considerations that mean at least one IP addess is needed.

  • To send a packet on an ethernet-like interface a MAC address is required, this MAC address is nearly always obtained by performing an arp (nd for ipv6) lookup on the "next-hop IP address" from the routing table. So to direct packets over and ethernet-like interface the recipiant must have an IP address.
  • Most modern dynamic routing protocols run on top of IP, so the router will need an IP address to implement those protocols.
  • Most routers are administered remotely, so the router will need an IP address for administration.
  • If something goes wrong (packet to big, destination unreachable etc) when routing a packet, then routers will often use ICMP to report this to the sender. This is particularly important in mixed-mtu networks. A source address is needed for ICMP packets.

The traditional way of configuring a router is to have seperate IP addresses for each interface, and a "loopback" IP that repreents the router regardless of what interfaces are currently operational. With some routers it is practical to run with only the loopback IP but it's really not very practical to run with no IP at all.

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    ICMP is that fundamental thing that requires IP routers to have an address. Not necessarily on every interface, but it needs at least one "unique" address to signal errors. (of course, one is free to run their router without add address, with a private address, and block all ICMP, but that would not be following "the rules".) Also, attempting to use the same address on more than one multi-access interface isn't routing, it's bridging. (and all the times I've seen some nut attempt it: a right bloody mess.)
    – Ricky
    Aug 22 at 22:59
  • "attempting to use the same address on more than one multi-access interface isn't routing, it's bridging" bridging is forwarding packets based on MAC addresses, routing is forwarding packets based on their IP addresses. Aug 23 at 21:16
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    BTW it's recently been proposed to allow IPv4 routes wit IPv6 nexthops ietf.org/id/draft-chroboczek-int-v4-via-v6-01.html . If the router has no IPv4 addresses they suggest using 192.0.0.8 as a source for ICMP packets, the same as in 4rd. Aug 23 at 21:25
  • It's RFC 9229 now <datatracker.ietf.org/doc/html/rfc9229>
    – jch
    Sep 30 at 20:09
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Routers route packets between networks, so they have addresses on the interfaces for each network to which they are directly connected. They may also have virtual interfaces, e.g. loopback, subinterfaces, etc., and each of those will have an address.

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  • P2P interfaces don't need an IP address. ;-)
    – Zac67
    Aug 22 at 16:20
  • Well, the Cisco PPP unassigned interface uses one of the other IP addresses assigned to a different interface, so it uses an IP address. That means the router needs an IP address (per the question).
    – Ron Maupin
    Aug 22 at 16:23
  • A PPP interface doesn't need an IP address of its own. You could have a working router with three PPP links and no IP address at all. Only for management does the router require an address of its own, but that is only a practical requirement.
    – Zac67
    Aug 22 at 16:35
  • With the IP Unnumbered feature configured, your interface shares an IP address of a different interface. On Cisco devices, if you do not configure an interface with an IP address then the IP protocol processing is deactivated on that interface, so it is incapable of sending/receiving IP packets. IP Unnumbered gets around that by sharing an IP address from a different interface, but the router must have an IP address on an interface to share.
    – Ron Maupin
    Aug 22 at 17:16
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    It is because of the way Cisco originally designed its routers. Each interface needed to be told which protocol(s) to use. An interface would not send or receive packets for any protocol unless you configured that protocol on the interface. That is done by setting an address for the interface, or, in the case of IP, unnumbered, which requires you to specify an interface with an IP address that the interface should use. You could configure IPX on an interface with an IPX address, and it will not pass IP packets unless you specify an IP address or use the ip unnumbered <interface> command.
    – Ron Maupin
    Aug 22 at 18:37

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