IP Unnumbered

IP Unnumbered
With IP unnumbered, a serial interface is not on a separate network, as most router interfaces
tend to be. Instead, the serial port “borrows” an IP address from another interface. In the following
router configuration example, interface Serial 0 is using a borrowed IP address from
interface Ethernet 0:
interface serial 0
ip unnumbered ethernet 0
Extending IP Addresses 65
Therefore, by using IP unnumbered, the apparently discontiguous subnets, shown in
Figure 2.4, are actually supported. Because the unnumbered interface shares another local
interface’s IP address, it is advised that a loopback interface be used, because virtual interfaces
never go down and the unnumbered interface will become inoperative should the associated
numbered interface go down.
IP unnumbered is compatible with point-to-point links only. An error will be returned if an
interface on a non-point-to-point segment, such as Ethernet, is attempted to be configured as
unnumbered.
There are a few things to be aware of before using IP unnumbered interfaces.
Because the serial interface has no unique IP address, you will not be able to
ping the interface to see if it is up, because the numbered interface will source
the echo reply, although you can determine the interface status with Simple
Network Management Protocol (SNMP). Nevertheless, pinging through the
unnumbered interface to one on the remote side from the echo source leads
us to believe the configuration is operational. In addition, IP security options
are not supported on an IP unnumbered interface. Due to the difficulty with
troubleshooting IP unnumbered interfaces, it is a practice that should be
avoided.