January 2002
Beginner
238 pages
6h 20m
English
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Variable Length Subnet Masks
Unlike RIP and IGRP, EIGRP updates carry
subnet mask information. The network
architect now has the responsibility of using addresses wisely.
Reviewing TraderMary’s configuration, a mask of
255.255.255.0 on the serial links is wasteful:
there are only two devices on the link, so a 24-bit mask will waste
252 addresses. A 30-bit mask (255.255.255.252)
allows two usable IP addresses in each subnet, which fits a serial
line exactly.
Let’s say that the network architect decided to
subdivide 172.16.250.0 using a 30-bit mask for use
on up to 64 possible subnets. The subnets that thus become available
are:
172.16.250.0172.16.250.4172.16.250.8...
172.16.250.252
The serial links in TraderMary’s network can be readdressed using these subnets:
hostname NewYork ... interface Ethernet0 ip address 172.16.1.1 255.255.255.0 ! interface Ethernet1 ip address 192.168.1.1 255.255.255.0 ! interface Serial0 description New York to Chicago linkip address 172.16.250.1 255.255.255.252! interface Serial1 description New York to Ames link bandwidth 56ip address 172.16.250.5 255.255.255.252... router eigrp 10 network 172.16.0.0 hostname Chicago ... interface Ethernet0 ip address 172.16.50.1 255.255.255.0 ! interface Serial0 description Chicago to New York linkip address 172.16.250.2 255.255.255.252! interface Serial1 description Chicago to Ames linkip address 172.16.250.9 255.255.255.0... router eigrp 10 network 172.16.0.0 hostname Ames ... interface Ethernet0 ip address ...
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