Sub-netting

It is the process of breaking the major network ( 10.0.0.0  /8 ) into multiple small parts is called Sub-Netting.

 

Benefits of Sub-nettings:

·       Decrease the wastage of unused IPs.

·       Security, by assigning the individual sub/network for each department ( Sales, Accounts, IT, Marketing).

 

How to create Sub/networks.

Sub-netting (FLSM) is the act of taking the bits from Host part & reserve them to increase the length of Network/ID. After doing this, it need to keep in mind very clear that the host part will shrink.

We will follow three step process to create Subnets.

1.  Determine ( Find) the number of bits, which will provide the required Sub-networks.

2.  Reserve number of bits into host part & find Increment (Magic Number).

3.  Use Increment to get Sub/Networks.

 

Que:  I have network 200.100.10.0 /24. I need to create 10    Sub/Networks.

*Determine ( Find) the number of bits, which will provide the required Sub-networks.

First find the number of bits which give 10 Sub/Networks.

   2*3 =  8             Note: Remember 2 is base & 3 is power.

   2*4 =   16

3 bits provide 8 Sub/Networks & 4 bits provide 16 Sub/Networks.

So number of bits = 4

 

* Reserve number of bits into host part & find Increment (Magic Number).

Current Mask =  255.255.255.0

In Binary        = 11111111.11111111.11111111.00000000

Reserve No. of  bits in host part

New Binary  = 11111111.11111111.11111111.11110000

In Decimal    = 255.255.255.240

Increment is the value of last reserved on bit in Host part.

So Increment   =   16

 

* Use Increment to get Sub/Networks.

   1st Sub-Network:       200.100.50.0      /28

   2nd Sub-Network:      200.100.50.16    /28

   3rd Sub-Network:       200.100.50.32    /28

   4th Sub-Network:       200.100.50.48    /28

   5th Sub-Network:       200.100.50.64    /28

   6th Sub-Network:       200.100.50.80    /28

   7th Sub-Network:       200.100.50.96    /28

   8th Sub-Network:       200.100.50.112  /28 

   9th Sub-Network:       200.100.50.128  /28

  10th Sub-Network:      200.100.50.144  /28

  11th Sub-Network:      200.100.50.160  /28

  12th Sub-Network:      200.100.50.176  /28

  13th Sub-Network:      200.100.50.192  /28

  14th Sub-Network:      200.100.50.208  /28

  15th Sub-Network:      200.100.50.224  /28

  16th Sub-Network:      200.100.50.240  /28

 

 

Find the First valid IP of first Sub/Network

Ans:     200.100.50.1

………………………

Find the Broadcast address of First Sub/Network

Ans:     200.100.20.15

               Because the Next Sub/Network is 200.100.50.16

……………..

Find the Last IP address of First Sub/Network

Ans:     200.100.50.14

……………..

Find the Total Available IP in First Sub/Network

Ans: How many host bits available after reserving the borrowed bits   =   4

=              2*4 – 2 =  16 – 2 = 14

14  IP addresses are available inside each Sub/Network.