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Cisco IOS in a Nutshell, 2nd Edition by James Boney

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Name

ip unnumbered — interface

Synopsis

ip unnumbered interface
no ip unnumbered interface

Configures

Interface IP address

Default

None

Description

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.

The 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.

Example

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 async2:

interface loopback 0
    ip address 10.10.1.4 255.255.255.0
interface async2
   ip unnumbered loopback0

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