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344 Chapter 5: Network Protocols
Answers to Scenario 5-3: IP Subnet Design with a Class
C Network
Planning the network in this scenario requires a topology that includes four sites, one Ethernet
at each site, and partially meshed Frame Relay for connectivity, as shown previously in Figure
5-45. The number of subnets will never grow. You must choose a mask that will maximize the
number of hosts per subnet, and you must use network 200.1.1.0.
Answers to Task 1 for Scenario 5-3
Given the design criteria and the network setup illustrated in Figure 5-45, this scenario requires
tricky subnet masks because a Class C network is used and because subnetting is needed. Using
Frame Relay subinterfaces, there will be a need for seven different subnets--one for each
Ethernet and one for each Frame Relay VC.
If 3 subnet bits are used, eight mathematical possibilities exist for subnet numbers. However,
one is the zero subnet and the other is the broadcast subnet. In this case, use of one or the other
is desired because the design called for maximizing the number of hosts per subnet. Deciding
against use of the zero and broadcast subnets then would require 4 subnet bits, leaving only 4
host bits, implying 14 hosts per subnet. So, 3 subnet bits and 5 host bits will be used in this
solution (mask of 255.255.255.224). Figure 5-51 summarizes the subnets on the network
diagram.
!
router igrp 6
network 172.16.0.0
Example 5-34
Router C Configuration, Scenario 5-2
ip subnet-zero
no ip domain-lookup
!
interface serial0
ip address 172.16.8.3 255.255.254.0
interface serial 1
ip address 172.16.10.3 255.255.254.0
interface ethernet 0
ip address 172.16.6.3 255.255.254.0
!
router igrp 6
network 172.16.0.0
Example 5-33
Router B Configuration, Scenario 5-2 (Continued)
ch05.fm Page 344 Monday, March 20, 2000 5:06 PM