Variable Length Subnetting

Variable Length SubnettingVariable Length Subnetting

Jim BlancoJim Blanco

Aparicio-Levy Technical CenterAparicio-Levy Technical Center

Variable Length SubnettingVariable Length Subnetting

VLSM allows us to use one class C VLSM allows us to use one class C address to design a networking scheme to address to design a networking scheme to meet the following requirements:meet the following requirements:

Perth Perth 60 users60 usersKula Lumpur Kula Lumpur 28 users28 usersSydneySydney 12 users12 usersSingaporeSingapore 12 users12 usersWAN links between each routerWAN links between each router

Networking Requirements

Kula 60

Perth 60

Sydney 60Sing. 60

WAN 2WAN 2

•Classful subnetting is very wasteful. •In the example above, a /26 was used to provide the 60 addresses for Perth and the other LANs. There are no addresses left for WAN links

VLSMVLSM

Forget everything you learned about Forget everything you learned about classfull subnetting. classfull subnetting.

Just remember you have 256 – 2 = 254 Just remember you have 256 – 2 = 254 addresses to work with.addresses to work with.

VLSMVLSM

Name/requiredName/required

addressesaddresses

Subnet Subnet addressaddress

Address rangeAddress range BroadcastBroadcast Address\subnetAddress\subnet

Perth/60Perth/60 192.168.12.0192.168.12.0

Kula/28Kula/28

Sydney/12Sydney/12

Sing/12Sing/12

WAN1/2WAN1/2

WAN2/2WAN2/2

WAN3\2WAN3\2

List your network requirements in descending order.

VLSMVLSM

addressesaddresses

Perth/60Perth/60 192.168.12.0192.168.12.0 .1 - 62.1 - 62 .63.63 192.168.12.0\26192.168.12.0\26

Kula/28Kula/28

Sydney/12Sydney/12

Sing/12Sing/12

WAN1/2WAN1/2

WAN2/2WAN2/2

WAN3\2WAN3\2

Calculate the subnet mask to meet largest requirement - Perth

VLSMVLSM

addressesaddresses

Perth/60Perth/60 192.168.12.0192.168.12.0 .1 - 62.1 - 62 .63.63 192.168.12.0\26192.168.12.0\26

Kula/28Kula/28 192.168.12.192.168.12.64 .65 - .94.65 - .94 .95.95 192.168.12.64\27192.168.12.64\27

Sydney/12Sydney/12

Sing/12Sing/12

WAN1/2WAN1/2

WAN2/2WAN2/2

WAN3\2WAN3\2

Keep track of subnet and broadcast addresses. Use the next available Address .64 to calculate a subnet mask for the next largest requirement - Kula Lumpur.

VLSMVLSM

addressesaddresses

Perth/60Perth/60 192.168.12.0192.168.12.0 .1 - 62.1 - 62 .63.63 192.168.12.0\26192.168.12.0\26

Kula/28Kula/28 192.168.12.64192.168.12.64 .65 - .94.65 - .94 .95.95 192.168.12.64\27192.168.12.64\27

Sydney/12Sydney/12 192.168.12192.168.12..96 .97- .110.97- .110 .111.111 192.168.12.96\28192.168.12.96\28

Sing/12Sing/12

WAN1/2WAN1/2

WAN2/2WAN2/2

WAN3\2WAN3\2

Sydney needs 12 addresses. Use the next available address .96 to calculatea subnet for Sydney’s requirement for 12

VLSMVLSM

addressesaddresses

Perth/60Perth/60 192.168.12.0192.168.12.0 .1 - 62.1 - 62 .63.63 192.168.12.0\26192.168.12.0\26

Kula/28Kula/28 192.168.12.64192.168.12.64 .65 - .94.65 - .94 .95.95 192.168.12.64\27192.168.12.64\27

Sydney/12Sydney/12 192.168.12.96192.168.12.96 .97- .110.97- .110 .111.111 192.168.12.96\28192.168.12.96\28

Sing/12Sing/12 192.168.12.192.168.12.112 .113 - .126.113 - .126 .127.127 192.168.12.112\28192.168.12.112\28

WAN1/2WAN1/2

WAN2/2WAN2/2

WAN3\2WAN3\2

Singapore also requires 12

VLSMVLSM

addressesaddresses

Perth/60Perth/60 192.168.12.0192.168.12.0 .1 - 62.1 - 62 .63.63 192.168.12.0\26192.168.12.0\26

Kula/28Kula/28 192.168.12.64192.168.12.64 .65 - .94.65 - .94 .95.95 192.168.12.64\27192.168.12.64\27

Sydney/12Sydney/12 192.168.12.96192.168.12.96 .97- .110.97- .110 .111.111 192.168.12.96\28192.168.12.96\28

Sing/12Sing/12 192.168.12.112192.168.12.112 .113 - .126.113 - .126 .127.127 192.168.12.112\28192.168.12.112\28

WAN1/2WAN1/2 192.168.12.192.168.12.128 .129 - .130 .129 - .130 .131.131 192.168.12.128\30192.168.12.128\30

WAN2/2WAN2/2

WAN3\2WAN3\2

WAN links require 2 addresses each

VLSMVLSM

addressesaddresses

Perth/60Perth/60 192.168.12.0192.168.12.0 .1 - 62.1 - 62 .63.63 192.168.12.0\26192.168.12.0\26

Kula/28Kula/28 192.168.12.64192.168.12.64 .65 - .94.65 - .94 .95.95 192.168.12.64\27192.168.12.64\27

Sydney/12Sydney/12 192.168.12.96192.168.12.96 .97- .110.97- .110 .111.111 192.168.12.96\28192.168.12.96\28

Sing/12Sing/12 192.168.12.112192.168.12.112 .113 - .126.113 - .126 .127.127 192.168.12.112\28192.168.12.112\28

WAN1/2WAN1/2 192.168.12.128192.168.12.128 .129 - .130 .129 - .130 .131.131 192.168.12.128\30192.168.12.128\30

WAN2/2WAN2/2 192.168.12.192.168.12.132 .133 - 134.133 - 134 .135.135 192.168.12.132\30192.168.12.132\30

WAN3\2WAN3\2

VLSMVLSM

addressesaddresses

Perth/60Perth/60 192.168.12.0192.168.12.0 .1 - 62.1 - 62 .63.63 192.168.12.0\26192.168.12.0\26

Kula/28Kula/28 192.168.12.64192.168.12.64 .65 - .94.65 - .94 .95.95 192.168.12.64\27192.168.12.64\27

Sydney/12Sydney/12 192.168.12.96192.168.12.96 .97- .110.97- .110 .111.111 192.168.12.96\28192.168.12.96\28

Sing/12Sing/12 192.168.12.112192.168.12.112 .113 - .126.113 - .126 .127.127 192.168.12.112\28192.168.12.112\28

WAN1/2WAN1/2 192.168.12.128192.168.12.128 .129 - .130 .129 - .130 .131.131 192.168.12.128\30192.168.12.128\30

WAN2/2WAN2/2 192.168.12.132192.168.12.132 .133 - 134.133 - 134 .135.135 192.168.12.132\30192.168.12.132\30

WAN3\2WAN3\2 192.168.12.192.168.12.136 .137 - .138.137 - .138 .139.139 192.168.12.136\30192.168.12.136\30

The networking problem is solved

Networking Scheme

Kula 192.168.10.64/27

Perth 192.168.10.0/26

Sydney 192.168.10.96/28Sing. 192.168.10.112/28

WAN 192.198.10.129 and 130WAN 192.198.10.133 and 134

WAN 192.198.10.137 and 138

Apply the address to the LAN or WAN link

VLSMVLSM

•A class C address such as 192.168.12.0 contains 256 - 2 = 254 usable addresses

•You used only addresses .0 - .139

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