Subnetting allows dividing a single network into multiple subnets. Each subnet is treated as a separate network and can be a LAN or WAN. Subnetting transforms host bits in the IP address into network bits, creating additional network identifiers from a single address block. The default subnet masks divide networks into classes A, B, and C. An example shows subnetting a Class C network with address 192.168.1.0/24 to create two /25 networks with 126 hosts each by using 1 host bit as a network bit. Transforming 2 host bits creates four /26 networks each with 62 hosts.

1. Subnetting

2. Subnet
 Subnets are a subset of the entire
network
 Networks can be divided into subnets
 Each subnet is treated as a separate
network
 A subnet can be a WAN or LAN

3. Subnets
Subnet 1 Subnet 2
Subnet 3 Subnet 4

4. Sub netting
o What?
Sub netting is the strategy that allows creating
multiple networks from a single one.
o Why?
Maximize addressing efficiency
Extend the life of IPv4
o How?
Transform host bits into network bits thus
creating additional networks from single address
block

5. Cont…
oThe default subnet mask is as follows:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0

6. Sub Netting in Class C
◦ Subnetting base address 192.168.1.0/24
11111111 11111111 11111111 00000000
◦ Transform 1 host bit into network bit
11111111 11111111 11111111 10000000
2 𝑏
networks (where b=no. host bits transformed) =2 networks
2ℎ
-2 hosts/ips per new networks (where h=no. of host bits left)=126 hosts
New prefix length /25
255.255.255.128
◦ So the first network will have the network id of
.0.25
◦ And the 2nd network will have the network id of
.128.25

7. Cont…
◦ Transform 2 host bits into networks bit
11111111 11111111 11111111 11000000
New prefix length /26
255.255.255.192
◦ So the first network will have the network id of
.0.26
◦ 2nd network:
.64.26
◦ 3rd network :
.128.26
◦ 4th network:
.192.26
And in each network we will have maximum 62 ips.

8. Subnet Masks
Mask No. of hosts Host Bits
255.255.255.0 254 bits 8
255.255.255.128 126 bits 7
255.255.255.192 62 bits 6
255.255.255.224 30 bits 5
255.255.255.240 14 bits 4
255.255.255.248 6 bits 3
255.255.255.252 2 bits 2
𝑈𝑠𝑖𝑛𝑔 𝑡ℎ𝑒 𝑓𝑜𝑟𝑚𝑢𝑙𝑎 ∶ 2ℎ
-2

9. Example
Subnet Base Address: 200.10.21.0/24
◦ Objective:
The network segment requires a maximum of 25 ip addresses.
Subnet the base address in order to yield the maximum address
utilization.
◦ Solution:
2ℎ
-2 => 25 (what is the smallest value of h that meets the criteria?)
h=5
b=32-(n+h)
b=32-(24+5)
b=3
2 𝑏
= 8 (networks) each having 30 ips
◦ New Mask:
255.255.255.224/27