Proxy Arp

[jwc2919]

Hi All

Please can you help with a proxy arp question.

Host A and Host B have ip addresses 10.2.1.2/16 and 10.3.1.2/16 respectively. Host A wants to send data to Host B and arps for host B's mac address. A kindly router sitting between the two subnet sends a proxy arp response to host A telling A to sed the data to the router's interface on the 10.2.0.0/16 subnet. The router relays it to Host B. Great.

But why does Host A arp for something on a different subnet? Why not just do the normal routing thing and arp for the default gateway?

Damn I thought I was getting the hang of routing.

Thanks in advance.

Jeff

[Fuzz]

Proxy ARP essentially hides subnet information from hosts, providing a transparent method of segmenting a network into separate broadcast domains. Imagine that hosts can only use classful routing in a subnetted network. They will be completely unaware that a destination host is on a different subnet.

Host A wants to send to Host B. It broadcasts an ARP request out, and the default gateway sees this request. As host B is on a different subnet, the gateway responds to host A's request by saying 'if you want to send something to host B, use my hardware address'. It is therefore acting as a proxy, because it sends back the MAC address of it's own interface, rather than the one that was requested by host A.