:: SUBNETTING ::

1. Defination

- Subnetting is the process of breaking down an IP network into smaller sub-networks called "subnets."

- Each subnet is a non-physical description (or ID) for a physical sub-network (usually a switched network of host containing a single router in a multi-router network).

2. Function

- Subnets are created to limit the scope of broadcast traffic, to apply network security measures, to separate network segments by function, and/or to assist in resolving network congestion problems.

- A subnet is usually composed of a network router, a switch or hub, and at least one host.

3. Count Subnetting

- There are 3 main classes of IP address that we are concerned with:-

• Class A Range 0 - 127 in the first octet (0 and 127 are reserved)
• Class B Range 128 - 191 in the first octet
• Class C Range 192 - 223 in the first octet

- Below shows you how, for each class, the address is split in terms of network (N) and host (H) portions.

• NNNNNNNN . HHHHHHHH . HHHHHHHH . HHHHHHHH Class A Address
• NNNNNNNN . NNNNNNNN . HHHHHHHH . HHHHHHHH Class B Address
• NNNNNNNN . NNNNNNNN . NNNNNNNN . HHHHHHHH Class C Address

- Format :-

- Exmple 1 : What subnet does 192.168.12.78/29 belong to?

• You may wonder where to begin. Well to start with let's find the next boundary of this address.
• Our mask is a /29. The next boundary is 32. So 32 - 29 = 3. Now 2 ^ 3 = 8 which gives us our block size.
• We have borrowed from the last octet as the 29th bit is in the last octet. We start from zero and count up in our block size. Therefore it follows that the subnets are:-
  

192.168.12.0

192.168.12.8

192.168.12.16

192.168.12.24

192.168.12.32

192.168.12.40

192.168.12.48

192.168.12.56

192.168.12.64

192.168.12.72

192.168.12.80.............etc

--> Our address is 192.168.12.78 so it must sit on the 192.168.12.72 subnet.

- Exmple 2 : What subnet does 172.16.116.4/19 sit on?

• Our mask is /19 and our next boundary is 24. Therefore 24 - 19 = 5. The block size is 2 ^ 5 = 32.
• We have borrowed into the third octet as bit 19 is in the third octet so we count up our block size in that octet. The subnets are:-

172.16.0.0

172.16.32.0

172.16.64.0

172.16.96.0

172.16.128.0

172.16.160.0.............etc

--> Our address is 172.16.116.4 so it must sit on the 172.16.96.0 subnet.