Example 1: Variable Length Subnet Mask (VLSM)

Example 1:

Example

Answer:

v  We need Extra 4 Hosts to Connect Two Routers 

i.e.Head Office to Branch 1 – 2 Host’s
Head Office to Branch 2     – 2 Host’s

Host Order Selection

v  So Totally we need 6 Subnets

v  Important to write Hosts in Descending Order

v  If we Choose CIDR, we Lost lot of IP’s

Ex: in Vlan 1, 100 Host needed. So Remaining 155 Waste. So we Choose VLSM

Vlan 4: (1000 Host)

Calculating Host's

So 10 bit's is enough to get 1000 host's

0th Subnet

v  We Never Use 0^th Subnet

Vlan-4 Subnet

Network ID : 172.16.4.0

1^st Host ID : 172.16.4.1

Last Host ID  : 172.16.7.254

Broadcast ID : 172.16.7.255

Subnet Mask   : 255.255.252.0

Vlan 2: (500 Host)

Calculating Host's

•   So 9 bit's is enough to get 500 host's

Vlan-2 Subnet

• We Put '1' in 4th bit Because Current Network ID is next IP of Last Vlan Broadcast ID

Network ID: 172.16.8.0

1^st Host ID: 172.16.8.1

Last Host ID: 172.16.9.254

Broadcast ID: 172.16.9.255

Subnet Mask: 255.255.254.0

Vlan 3: (200 Host)

Calculating Host's

•   So 8 bit's is enough to get 200 host's

Vlan-3 Subnet

• Current Network ID is next IP of Last Vlan Broadcast ID

Network ID: 172.16.10.0

1^st Host ID: 172.16.10.1

Last Host ID: 172.16.10.254

Broadcast ID: 172.16.10.255

Subnet Mask: 255.255.255.0

Vlan 1: (100 Host)

Calculating Host's

• So 7 bit's is enough to get 100 host's

Vlan-1 Subnet

• Current Network ID is next IP of Last Vlan Broadcast ID

Network ID: 172.16.11.0

1^st Host ID: 172.16.11.1

Last Host ID: 172.16.11.126

Broadcast ID: 172.16.11.127

Subnet Mask: 255.255.255.128

Head to Br1: (2 Hosts)

Calculating Host's

• If We Select 1 Bit,then We Can't Get 2 Host IP's, So Take 2 Bits as a Host. 

For Example We will try to Calculate Host by taking 1 Bit

Calculating Host's by using 1 Bit

• Finally We Can't Get 2 Host's
• So We Will Select 2 Bit's to Get 2 Host's

Calculating Host's by using 2 Bit's

Network ID: 172.16.11.128

1^st Host ID: 172.16.11.129

Last Host ID: 172.16.11.130

Broadcast ID: 172.16.11.131

Subnet Mask: 255.255.255.252

Head to Br2: (2 Hosts)

Calculating Host's

Network ID: 172.16.11.132

1^st Host ID: 172.16.11.133

Last Host ID: 172.16.11.134

Broadcast ID: 172.16.11.135

Subnet Mask: 255.255.255.252