ip unnumbered — interface
interfaceno ip unnumbered
Interface IP address
Normally, creating point-to-point links requires dedicating a subnet specifically for the link. This works well if all your equipment supports variable-length subnet masks (VLSM), but can be very wasteful if your equipment doesn’t support VLSM. This forces you to assign relatively large subnets to your point-to-point links.
ip unnumbered command tells the router to use the IP address of the selected
interface as the address for this link. In other words, the router “borrows” the IP address of the named interface and uses that as the link’s address.
Assume that older equipment in our network forces us to use a subnet mask of 255.255.255.0. This means that assigning a subnet to a point-to-point link would use 254 addresses, of which only two are actually doing something. Instead of wasting 252 addresses, we can use the
ip unnumbered command to borrow the address of another interface for use on the serial link:
interface serial0 ip unnumbered ethernet0 encapsulation ppp clockrate 1300000
Borrowing the address of the loopback interface for an unnumbered interface is often a good idea because the loopback interface is always up. The following configuration uses the loopback interface to provide the IP address for interface
interface loopback 0 ip address 10.10.1.4 255.255.255.0 interface async2 ip unnumbered loopback0