Example 2: Variable Length Subnet Mask (VLSM)

Example2:

Example 2

172.16.0.0/16

Answer:

v    We need Extra 2 Host’s to Connect Two Routers

      i.e.       R1 to R2 – Two IP

                

Host Order Selection

v  So Totally we need 5 Subnets

v  Important to write Hosts in Descending Order

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

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

Vlan C: 1000 Host

Calculating Host's

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

0th Subnet

•   We Never Use 0^th Subnet

Vlan-C 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 B: 500 Host

Calculating Host's

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

Vlan B 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 A: 100 Host

Calculating Host's

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

Vlan-A Subnet

v  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: 172.16.10.126

Broadcast ID: 172.16.10.127

Subnet Mask: 255.255.255.128

Vlan D: 50 Host

Calculating Host's

v  So 6 bit's is enough to get 50 host's

Vlan - D Subnet

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

Network ID: 172.16.10.128

1^st Host ID: 172.16.10.129

Last Host ID: 172.16.10.190

Broadcast ID: 172.16.10.191

Subnet Mask: 255.255.255.192

R1 to R2 : 2 Host

Calculating Host's

v  So 2 bit's is enough to get 4 Host's

R1 to R2 Subnet

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

Network ID: 172.16.10.192

1^st Host ID: 172.16.10.193

Last Host ID: 172.16.10.194

Broadcast ID: 172.16.10.195

Subnet Mask: 255.255.255.252