AFAIK, unnumbered interfaces are used only on routers on serial links and
things like pos. Basically one would then enter a route entry routing one or
more ip-blocks via an interface, not via the ip of a neigbouring router, in
stead of the ip address of the neigbouring router one enters the name of the
interface to use for this route.
Since these interfaces have no ip address, they cannot be the source of ip
packets. Applications that generate ip traffic on such a router will use a
different ip address on that router a the source ip, even if the packet has to
go out on an unnumbered interface.
Having said this, unnumbered interfaces are quite rare in ip networks, most
backbones use ip addresses on all of the interfaces of their routers. (do a
traceroute to anywhere, you'll find very few hops that show no ip address).
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