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In the TCP/IP Volume 1, it is stated that ARP works only on the same subnet. Does this mean that the outgoing interface of a router is necessarily on the same subnet as the incoming interface of the NEXT HOP router. If not, it would be impossible to send the Layer 2 Frame to the appropriate NEXT HOP router interface as it would not be possible to perform an ARP request.

Please correct me if there is any flaw in my understanding.

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    Basically, yes. However, note that frames belong to L2. – Zac67 Jul 9 at 18:52
  • Think about it this way, at layer-2, everything is a host on the network. Layer-2 does not have routers, so a router is just another host, and for one host to talk to another host, it must happen at layer-2 with layer-2 addressing. – Ron Maupin Jul 9 at 20:19
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ARP is used by IPv4 on broadcast networks, and not all data-link connections are broadcast connections, and IPv6 does not use ARP.

What you need to remember is that any particular router link (other than something like PPP) may have many devices, including multiple routers, connected to that same link.

For something like ethernet and IPv4, then, yes, ARP would be used to resolve the IPv4 address to a MAC address. Otherwise, the router would not know how to address the data-link frame. The router needs to make sure that the frame gets delivered to the the correct next-hop router, and broadcasting would be a waste of network resources, interrupting every device on the broadcast network for every frame, rather than directing the frames to the proper destination.

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Each router needs to be able to talk to the next hop router. This requires them to either share a point-to-point link (including overlays like VPN), or a common network and subnet.

Note that IPv4 uses ARP only for MAC-based networks, there are others as well.

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