Subnetting

Technique to divide a subnet to more subnet

Dividing

For example, lets subnetting 10.10.10.0/24

To divide this subnet, we first increment the / by 1 and then divide the address by 2. So:

• 10.10.10.0/24
  1. 10.10.10.0/25
  2. 10.10.10.128/25
  • Note: /25 means 7 for hosts which allows 2^7 = 128 usable hosts

We can continue to divide like so:

• 10.10.10.0/24
  1. 10.10.10.0/25
     1. 10.10.10.0/26
     2. 10.10.10.64/26
  2. 10.10.10.128/25
     1. 10.10.10.128/26
     2. 10.10.10.192/26

Merging

For example merge the 2 subnets 10.10.0.0/24 and 10.10.1.0/24 we need to identify the binary format:

• 10.10.0.0/24
  • xxxxxxxx.xxxxxxxx.00000000.00000000
• 10.10.1.0/24
  • xxxxxxxx.xxxxxxxx.00000001.00000000

To merge these 2, we need the 24th bit to be flexible (either 1 or 0). Therefore, we can make it a /23.

• So we have a 10.10.0.0/23 can cover both

Similarly, if we have a

• 10.10.2.0/24
  • xxxxxxxx.xxxxxxxx.00000010.00000000
• 10.10.3.0/24
  • xxxxxxxx.xxxxxxxx.00000011.00000000

We can have the 24th bit as the change bit and prefix as 1 like:

So we can merge into a 10.10.2.0/23 which have

• xxxxxxxx.xxxxxxxx.0000001c.cccccccc
• where c is the flexible bit.

IPv6 subnet

The subnet part of the IPv6 address is the S bit in the middle

P bit is normally 32-48 bit
It's going to be 64 - P bit