2. Why use Proxy ARP with subnetting?

The applications for using Proxy ARP with subnetting are fairly specific.

In my case, I had a wireless Ethernet card that plugs into an 8-bit ISA slot. I wanted to use this card to provide connectivity for a number of machines at once. Being an ISA card, I could use it on a Linux machine, after I had written an appropriate device driver for it - this is the subject of another document. From here, it was only necessary to add a second Ethernet interface to the Linux machine and then use some mechanism to join the two networks together.

For the purposes of discussion, let network 0 be the local Ethernet connected to the Linux box via an NE-2000 clone Ethernet interface on eth0. Network 1 is the main network connected via the wireless Ethernet card on eth1. Machine A is the Linux box with both interfaces. Machine B is any TCP/IP machine on network 0 and machine C is likewise on network 1.

Normally, to provide the connectivity, I would have done one of the following:

In my case, getting a new subnet (network) number was not an option, so I wanted a solution that allowed all the machines on network 0 to appear as if they were on network 1. This is where Proxy ARP comes in. Other solutions are used to connect other (non-IP) protocols, such as netatalk to provide AppleTalk routing.