I'm running 2.2.5-RELEASE, and the system has one Ethernet
interface with several IP addresses.
Up until now, all the secondary addresses were on the same
subnet. I recently added a secondary address on a different subnet,
which is of course present on the same wire, and while the new address
is reachable from other hosts, I see kernel complains about the
default gateway for the new address being on a different subnet.
It would appear that when there are packets that need to go to
an address in the same subnet as the new secondary address, then
initial ARP is done by the primary address, the kernel complains, and
eventually the ARP is done by the corresponding secondary address.
I read the docs on IP aliasing, I didn't see any restrictions
mentioned about all the secondary addresses having to be in the same
subnet as the primary.
Just thought I'd mention this, in case someone else has seen