Actually, proxy-arp applies when an ARP request is received that asks for a
host that the router knows is not on the same wire by dint of network mask,
and for which it has a route (typically other than "default", though an
argument can be made for proxy-arp in that case too). The router replies with
its own Ethernet station address and everything works because the packets will
be routed correctly to their destination.

Proxy-arp was the recommended way to deal with hosts on a subnetted LAN which
did not themselves implement subnetting (since, not knowing the subnet mask,
they'll simply ARP for all hosts in the LAN), though, as a side effect, it
also took care of hosts that are simply misconfigured (i.e. default netmask
for the "natural" address class, as opposed to the typically longer subnet mask).

So, what I was asking about is a default behavior in a modern IP router, rather
than something explicitly "turned on" or configured through the "arp" command,
on a per-address basis.

Does that clarify my question a bit better?

	Erik E. Fair    fair@clock.org