ip unnumbered

iworms

Hi all, I just read about ip unnumbered and it seems unintuitive. So far in my CCNA learning (2+ weeks), I believe the two ends on a serial link need to be in the same subnet. I tried setting my two routes' serial interfaces (linked back-to-back) to different subnets, i.e. 172.16.0.1/30 and 172.16.0.5/30, and sure enough they can't ping each other.

Now if a serial interface is configured unnumbered, it won't be in the same network with its neighbor, but they can still communicate? I haven't tried it yet.

EDIT:
OK, I've done more poking around. It appears that the two ends on a point-to-point link (e.g. serial link) can have ip addresses belonging to different networks. They can communicate because, with the link being point-to-point involving two nodes, there will be no confusion about who the source or destination is.

The reason I could not ping from one to the other is because I had no routing set up (no routing protocol, no static entry). In this case if the two are in the same subnet, no problem. But when doing ip unnumbered or different subnets, a proper routing table is necessary.

Just my understanding so far. Could be wrong/incomplete.

networker050184

You can use ip unnumbered on a point-to-point link because the router uses a next hop of the exit interface when the other end of the link is using ip unnumbered. This would not be practical on a multi-access link.