by a protocol, to detirmine if an address matches the net this address
is part of. This is needed by protocols for which netmasks
"just don't work", for example appletalk.
Also add the code in appletalk to make use of this new feature.
Thsi fixes one of the longest standing bugs in appletalk.
The inability to talk to machines to which the path is via a router
which is on a different net, but the same netrange, as your interface.
Protocols that do not supply this function (e.g. IP) should not be affected.
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