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Presentation

Submitted BySharmin

Sultana JeesaID# 141-15-

3219Sec: D

ContentVariable Length Subnet Masks

(VLSM)

VLSMVariable Length Subnet Masks (VLSM) is a technique that network administrators employ to subnets a subnet.

Why VLSM?Basically, In normal subnetting we

can use only one subnet mask for the whole network that can be wasteful. But VLSM allow multiple subnet masks to exist within the one network.

So, VLSM is used for preventing the waste of IP addresses.

Steps for VLSMFind the segment which need largest number of

hosts address.Do subnetting to fulfill the requirement of largest

segment.Assign the appropriate subnet mask for the

largest segment.For second largest segments, take one of these

newly created subnets and apply a different, more appropriate, subnet mask to it.

Assign the appropriate subnet mask for the second largest segment.

Repeat this process until the last network.

ExampleAssume a network administrator of ABC software

have an network 192.168.1.0/24 and wants to assign IP among all the departments. Company have three departments connected with WAN links.

• Development department have 74 computers.• Production department have 52 computers.• Administrative department have 28 computers.• All departments are connected with each other

via WAN link.• Each WAN link requires two IP addresses.

SolutionAfter ordering all segments according

the hosts requirement (Largest to smallest).Subnet Segment Host

1 Development

74

2 Production

52

3 Administrative

28

4 WAN link 1

2

5 WAN link 2

2

6 WAN link 3

2

SolutionFor 1st subnet,

No. of hosts = 2⁷-2 =126 ≥ 74 ; n=7;So, mask= /25; Block size = 256-128 =

128; Similarly, for 2nd subnet,

Mask= /26; Block size = 64;For 3rd subnet,

Mask = /27; Block size = 32;For 4th, 5th & 6th subnet,

Mask = /30; Block size = 4;

For 1st subnet

1 Network254 hosts192.168.10.0/24

2 Subnets128 hosts each192.168.10.0/25192.168.10.128/25

Development

For 1st subnet Segment Development

Requirement 74CIDR /25

Subnet mask 255.255.255.128Network ID 192.168.10.0First hosts 192.168.10.1Last hosts 192.168.10.126

Broadcast ID 192.168.10.127

For 2nd subnet

2 Subnets128 hosts each192.168.10.0/25192.168.10.128/25

2 Subnets64 hosts each192.168.10.128/26192.168.10.192/26

Production

For 2nd subnet Segment Production

Requirement 52CIDR /26

Subnet mask 255.255.255.192Network ID 192.168.10.128First hosts 192.168.10.129Last hosts 192.168.10.190

Broadcast ID 192.168.10.191

For 3rd subnet

2 Subnets64 hosts each192.168.10.128/26192.168.10.192/26

2 Subnets32 hosts each192.168.10.192/27192.168.10.224/27

Administrative

For 3rd subnet Segment Administrative

Requirement 28CIDR /27

Subnet mask 255.255.255.224Network ID 192.168.10.192First hosts 192.168.10.193Last hosts 192.168.10.222

Broadcast ID 192.168.10.223

For 4th, 5th & 6th subnet

2 Subnets32 hosts each192.168.10.192/27192.168.10.224/27

8 Subnets4 hosts each192.168.10.224/27192.168.10.228/27192.168.10.232/27192.168.10.236/27192.168.10.240/27192.168.10.244/27192.168.10.248/27192.168.10.252/27

WAN link 1WAN link 2WAN link 3

For 4th subnet Segments Wan Link 1

Requirement 2CIDR /30

Subnet mask 255.255.255.252Network ID 192.168.10.224First hosts 192.168.10.225Last hosts 192.168.10.226

Broadcast ID 192.168.10.227

For 5th subnet Segments Wan Link 2

Requirement 2CIDR /30

Subnet mask 255.255.255.252Network ID 192.168.1.228First hosts 192.168.1.229Last hosts 192.168.1.230

Broadcast ID 192.168.1.231

For 6th subnet Segments Wan Link 3

Requirement 2CIDR /30

Subnet mask 255.255.255.252Network ID 192.168.1.232First hosts 192.168.1.233Last hosts 192.168.1.234

Broadcast ID 192.168.1.235

Thank You