Throughout this chapter,
you have seen ways to maximize an organization's use of IP
addresses. In previous sections, you learned that you could avoid
wasting an entire subnet
on the point-to-point serial links by using VLSM, or use private
addresses instead.
Neither technique can be supported by classful routing protocols,
such as the popular RIPv1 and IGRP. Fortunately, the Cisco IOS offers a third option for
efficiently
addressing serial links: IP unnumbered.
When a serial interface is configured for IP unnumbered, it borrows
the IP address of
another interface (usually a LAN interface or loopback interface)
and therefore does not
need its own address.
Not only does IP unnumbered avoid wasting
addresses on point-to-point WAN links, but it also can be used with classful routing
protocols, where VLSM and discontiguous subnets cannot. If your network runs RIPv1 or IGRP,
IP unnumbered
may be the only solution to maximize your addresses.
RTA's S1 (168.71.5.1) and RTB's S1 (168.71.8.1) can communicate
using TCP/IP over
this serial link, even though they do not belong to the same IP
network.
This is possible
because it is a point-to-point link, so there is no confusion about
which device a packet is
originating from or destined for. There are two ground rules for
configuring IP unnumbered on an interface:
- The interface is both serial and
connected via a point-to-point link.
- The same major network with the same
mask is used to address the LAN
interfaces that "lend" their IP address on both sides of
the WAN link.
OR
Different major networks with no subnetting are used to address the
LAN interfaces on both sides of the WAN link.
Using IP unnumbered is not without its drawbacks, which include the
following:
- You cannot use
ping
to
determine whether the interface is up because the interface has
no IP address.
- You cannot boot from a network IOS
image over an unnumbered serial interface.
- You cannot support IP security
options on an unnumbered interface.
|