Name
ip unnumbered — interface
Synopsis
ip unnumberedinterface
no ip unnumberedinterface
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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