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Regarding "Why are routers not answering ARP Broadcasts?"

As per my knowledge if the target network is different subnet and there is no default gateway is configured the packet will not be constructed at that Host A itself?

Can anyone explain this in detail?

How does it broadcast the ARP request by asking the Router's MAC address?

Note : In my case I statically assigned an IP address to Host-A and I didn't configure the default gateway.

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  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Aug 8, 2017 at 15:26

1 Answer 1

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Say Host A wants to talk to Host B, yet they're in different networks and neither has a route to the other (no specific route or default route.) This generates an immediate error ("Destination Network Unreachable"), with zero network traffic.

If you're doing Proxy ARP, then you do have a default route; the destination is an interface instead of an address. (or your netmask is 0.0.0.0, but I don't think anybody allows that) In this case, an ARP is sent to the all-one's broadcast address (255.255.255.255) and then awaits an answer from anyone on the wire. Presumably, a router on the wire will answer and forward traffic in the correct direction. If not, the request times out and error is generated.

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  • The ARP broadcast is actually L2 isn't it, ie ffff.ffff.ffff?
    – John Jensen
    Dec 3, 2013 at 5:04
  • All broadcast is ff-ff-ff-ff-ff-ff at layer-2. I'm talking about the layer-3 ARP payload.
    – Ricky
    Dec 3, 2013 at 5:26
  • If we have a default route means , the proxyARP done only once right ? I mean the ARP resolution of the default gateway ...
    – user2720323
    Dec 3, 2013 at 6:24
  • Until the entry in the ARP cache expires, yes.
    – Ricky
    Dec 3, 2013 at 7:14
  • @RickyBeam: ARP runs directly over Ethernet, not over IP: you can't send an ARP request to 255.255.255.255 requesting host B's address, because there is no destination IP address field in the packet to put 255.255.255.255 in :)
    – psmears
    Nov 28, 2014 at 21:00

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