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Configuring VLSM


Configuring VLSM
1.1.6 This page will teach students how to calculate and configure VLSM. If VLSM is the scheme chosen, it must then be calculated and configured correctly. 
The following are VLSM calculations for the LAN connections in Figure :
  • Network address: 192.168.10.0
  • The Perth router has to support 60 hosts. That means a minimum of six bits are needed in the host portion of the address. Six bits will yield 26 – 2, or 62 possible host addresses. The LAN connection for the Perth router is assigned the 192.168.10.0/26 subnet.
  • The Sydney and Singapore routers have to support 12 hosts each. That means a minimum of four bits are needed in the host portion of the address. Four bits will yield 24 – 2, or 14 possible host addresses. The LAN connection for the Sydney router is assigned the 192.168.10.96/28 subnet and the LAN connection for the Singapore router is assigned the 192.168.10.112/28 subnet.
  • The KL router has to support 28 hosts. That means a minimum of five bits are needed in the host portion of the address. Five bits will yield 25 – 2, or 30 possible host addresses. The LAN connection for the KL router is assigned the 192.168.10.64/27 subnet.
The following are VLSM calculations for the point-to-point connections in Figure :
  • Perth to KL
The connection from Perth to KL requires only two host addresses. That means a minimum of two bits are needed in the host portion of the address. Two bits will yield 22 – 2, or 2 possible host addresses. The Perth to KL connection is assigned the 192.168.10.128/30 subnet.
  • Sydney to KL
The connection from Sydney to KL requires only two host addresses. That means a minimum of two bits are needed in the host portion of the address. Two bits will yield 22 – 2, or 2 possible host addresses. The Sydney to KL connection is assigned the 192.168.10.132/30 subnet.
  • Singapore to KL
The connection from Singapore to KL requires only two host addresses. That means a minimum of two bits are needed in the host portion of the address. Two bits will yield 22 – 2, or 2 possible host addresses. The Singapore to KL connection is assigned the 192.168.10.136/30 subnet.
The following configuration is for the Singapore to KL point-to-point connection: 
Singapore(config)#interface serial 0
Singapore(config-if)#ip address 192.168.10.137 255.255.255.252
KualaLumpur(config)#interface serial 1
KualaLumpur(config-if)#ip address 192.168.10.138 255.255.255.252
This page concludes this lesson. The next lesson will discuss RIP. The first page describes RIP v1.

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