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Re: ifconfig v2

> the interesting case is, what happens when you have two interfaces
> with addresses/netmasks

> (0xffff00ff)
>        (0xffff0ff0)

> and you send a packet to ?

It's on-net for neither interface, so, unless there's another interface
that it is on-net for, the normal "not on an attached net" routing
mechanisms control what happens.  (More interesting would be a packet
to, which may even have been what you meant.)

If the destination is on-net for fewer than two interfaces, there's no
ambiguity: either it goes out the interface it's on-net for, or there
is no such interface, in which case the routing table controls.  If the
destination is on-net for multiple interfaces, I see two reasonable
approaches offhand (pick one, or send to all); the only respect in
which this is even slightly more difficult than the same case with
contiguous netmasks is that here, neither one is a subnet of the other.
I see no problem with making it formally undefined, with actual
behaviour a matter of quality-of-implementation and/or local policy.
That's what we've had in practice on any implementation supporting
noncontiguous netmasks anyway.

> What about

> (0xffff08ff)
>        (0xffff0ff1)    ?

With the same destination?  Again, it's on-net for neither interface,
so there's no issue here as far as I can see - or, for, the
same remarks apply.

> (Yes, I must admit that I've seen one use of them where only one
> interface was involved, and the situation on the router wasn't
> ambiguous.)

My own case was similar; I ran a (small but) production network with a
netmask of (0xffffffd8) for years.  Except for the
router to the world, everything had only one interface, and, on that
router, the situation was entirely unambiguous.  (I think the rest of
its containing network routed it as if it were two /29s, but that
detail is lost to the mists of memory.)

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