IP Subnetting (VLSM) Step by Step

Main Network: 172.20.0.0/16

Required:

·         Switch 0: 225 host

·         Switch 1: 155 host

·         Switch 2: 12 host

·         Switch 3:16 host

·         Router1-Router0: 2 hosts

 

Switch 0: 225 host

172.20.0.0/16 >> formula host calculation: 2ⁿ-2 where n= bits borrowed from host part

Hosts=2⁷-2 = 128-2=126 no satisfying our requirement

So, Hosts= 2⁸-2= 256-2= 252, To satisfy the host requirement of 225 155 we must dedicate 8 bits from the last octet for hosts and the remaining bits are added as network bits.

The new subnet can be written as 172.20.0.0/16+8= 172.20.0.0/24

Switch 1: 155 host

172.20.0.0/16 >> formula host calculation: 2ⁿ-2 where n= bits borrowed from host part

Hosts=2⁷-2 = 128-2=126 no satisfying our requirement

So, Hosts= 2⁸-2= 256-2= 252, To satisfy host requirement of 155 we must dedicate 8 bits from the last octet for hosts and the remaining bits are added as network bits.

The new subnet can be written as 172.20.1.0/16+8= 172.20.1.0/24

Switch 3: 16 host

172.20.0.0/16 >> formula host calculation: 2ⁿ-2 where n= bits borrowed from host part

Hosts=2⁴-2 = 16-2=14 no satisfying our requirement

So, Hosts= 2⁵-2= 32-2= 30, To satisfy the host requirement of 16 we must dedicate 5 bits from the last octet for the host part and the remaining 3 bits are added as network bits.

The new subnet can be written as 172.20.2.0/16+8+3= 172.20.2.0/27

Switch 2: 12 host

172.20.0.0/16 >> formula host calculation: 2ⁿ-2 where n= bits borrowed from host part

Hosts=2³-2 = 8-2=6 not satisfying our requirement

So, Hosts= 2⁴-2= 16-2= 14, To satisfy host requirement of 14 we must dedicate 4 bits from the last octet for the host part and the remaining 4 bits are added as network bits.

The new subnet can be written as 172.20.2.32/16+8+4= 172.20.2.32/28

Router-Router: 2 host

172.20.0.0/16 >> formula host calculation: 2ⁿ-2 where n= bits borrowed from host part

Hosts=2¹-2 = 0 not satisfying our requirement

So, Hosts= 2²-2= 4-2= 2, To satisfy the host requirement of 2 we must dedicate 2 bits from the last octet for the host part and the remaining 6 bits are added as network bits.

The new subnet can be written as 172.20.2.48/16+8+6= 172.20.2.48/30

 

Subnet Table

Required Subnet	Required Host Number	Assigned Host Number	Address	Dec Mask	Assignable Range
Switch1	225	254	172.20.0.0/24	255.255.255.0	172.20.0.1 - 172.20.0.254
Switch0	155	254	172.20.1.0/24	255.255.255.0	172.20.1.1 - 172.20.1.254
Switch3	16	30	172.20.2.0/27	255.255.255.224	172.20.2.1 - 172.20.2.30
Switch2	12	14	172.20.2.32/28	255.255.255.240	172.20.2.33 - 172.20.2.46
Router	2	2	172.20.2.48/28	255.255.255.252	172.20.2.49 - 172.20.2.50