Subnetting

SUBNETTING

What is subnetting ?

Sub netting allows creating multiple networks from a single address block.

Why do we use subnetting ?

We use sub netting for:-
(a) Maximize addressing efficiency.
(b) Extend the life of IPv4.
(c)Public IPv4 addresses are scarce.

How do we use subnetting ?

We use it for transforming host bits into network bits creating additional networks from a single address block.

Formula and Variables

h – Number of host bits.
b – Number of host bits transfer into network bits.
n – Number of network bits.
2^b – Number of networks resulting from the subnetting.
2^h-2 – Number of hosts per new network. 

Example of subnetting:-

Subnetting base address – 192.168.1.0/24
11111111   11111111   11111111   00000000

Transform 1 host bit into a network bit.
11111111   11111111   11111111   10000000
2^b networks
2^h-2 hosts per new network.
New prefix length /25
     255.255.255.128

How we find no. of hosts in 1 bit?

11111111   11111111   11111111   1|0000000
                                    Subnet mask – 255.255.255.128

2^7-2= 126
255.255.255.0/25 – Network ID
255.255.255.128/25 – Broadcast ID 

Transform 2 host bits into network bits.
11111111   11111111   11111111   11000000
2^b network
2^h-2 host per new network
New prefix length /26     255.255.255.192

How we find no. of hosts in 2 bit?

11111111   11111111   11111111   11|0000000
                                    Subnet mask – 255.255.255.192

2^6-2= 62
255.255.255.0/26 – Network ID.
255.255.255.64/26 – Network ID.
255.255.255.128/26 – Network ID.
255.255.255.192/26 – Broadcast ID.  

	SUBNET MASK
Mask	Number of Hosts	Host Bits
255.255.255.0	254	8
255.255.255.128	126	7
255.255.255.192	62	6
255.255.255.224	30	5
255.255.255.240	14	4
255.255.255.248	6	3
255.255.255.252	2	2