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I understand the idea of proxy arp, that it allows me to communicate with other computers from other subnet, because proxy broadcast is made and router act as a proxy, but what if I disable proxy arp ? For example, I have empty arp table and I want to ping computer B. Then arp broadcast is made , but router doesn't redirect it so I won't be able to communicate with computer from others subnet ?

I know that it might be very basic question, but after reading about it I am still not able to understand it clearly. I read that having arp proxy enabled can hide subnet misconfiguration. Could someone give me an example ?

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Let's start out with the remark that routers not doing proxy arp is the default situation, proxy arp is disabled in most networks, and, if not used carefully, it can break a lot of things in nasty ways.

In the normal situation with proxy ARP disabled your client will see that the other machine is not on its local network. So it looks at its routing table and see that it should reach the other network via the router. Then, it will forward packets there.

As for your second question: one example of that is a misconfiguration in netmasks. A machine with 10.0.0.1/24 can think it's netmask was configured as a /16 because it sees ARP entries for 10.0.1.1/16, while the latter is not on the local network.

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  • Will a device update it's assigned (static or DCHP) netmask because of an arp request received for another network? Or does proxy-arp cover the subnet misconfiguration by allowing a host with a larger subnet configured than the gateway router to arp for devices that it thinks are local to it's network and the gateways proxy-arp config permits a response?
    – cpt_fink
    Dec 2 '14 at 23:43
  • A machine should never updat its netmask based on ARPs from other hosts, it would break networks in horrible ways. Your second statement is one of the possible ways proxy ARP can hide misconfiguration.
    – Teun Vink
    Dec 3 '14 at 6:08
  • ARP requests don't include the mask. If a machine on the 10.0.0.1/24 network sees an ARP Request for 10.0.1.1 (which is indeed on a different subnet), it will simply ignore it, just as it would if it receives an ARP Request for 10.0.0.50 (which is an IP in its own subnet). The original host will surely not think anything of the other party's netmask.
    – Eddie
    Dec 3 '14 at 17:24
  • That was my point... the answer to the second question is incorrect.
    – cpt_fink
    Dec 3 '14 at 20:22

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