IP unnumbered
refers to using the network or subnet address of a local LAN interface
as the router's network or subnetwork address for a point-to-point serial
link. The term point-to-point means that only two devices are on
the link, as is the case on a T1 connection between two routers or a
point-to-point subinterface to a Frame Relay network. Normally, a serial
link has its own unique network or subnetwork address. IP unnumbered
allows a network administrator to conserve network or subnetwork
addresses. IP unnumbered is especially valuable for networks running IP
routing protocols such as RIP V1 and IGRP, which do not support VLSM.
Without VLSM support, a network must use the same subnet mask on its
serial (WAN) interfaces as it does on its LAN interfaces. For a company
using an 24-bit (255.255.255.0) mask on a Class B address, this means
applying a subnet capable of supporting 254 hosts on a WAN link that never
has more than two hosts.
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Lab
Activity |
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In
this lab, you will learn how improper IP
addressing can have a negative impact on a network and the use of IP
Unnumbered on a point to point serial link to save IP addresses. |
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One of the reasons IP unnumbered is
possible is that with only two hosts on any serial link --- when the routers
are at either end --- there is no confusion about which device a packet is
originating from or destined for. The following are a few rules that apply
to using IP unnumbered:
- The serial interface must be a
point-to-point link.
- You must use the same major network with
the same mask on both sides of the WAN link.
or
- You must use different major nets with
no subnetting on both sides of the WAN link.
These rules are discussed in more detail in
the following sections.
Configuring IP Unnumbered on Serial Interfaces
Configuring IP unnumbered for RIP and IGRP
is done in the same way as explained previously. The excerpts from the
configurations of RouterA and RouterC show how to configure IP
unnumbered on a serial interface. (see RouterA#show
running-config command output).
The show interface commands
from RouterA and RouterC show that IP unnumbered is now in use on both
serial interfaces. (see RouterA#show interface serial 1
command output).
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