Subnet Summarization

One of the simplest concepts in networking is subnetting. Yet I always see people struggle with it, and mainly route summarization.

The process of summarizing routes comes from 2 simple points:

  • Maintaining several static routes can be a pain
  • It saves router memory

The easiest way to do route summarization is to just memorize these magic numbers.

128 192 224 240 248 252 254 255

128  64   32   16     8       4     2     1

Now say for example we have a /24 of 192.168.1.0 255.255.255.0 which gives us 255 addresses (yes I’m counting the network and broadcast in there)

Let’s make it a /25 just take a bit a way from the hosts and now you have 192.168.1.0 255.255.255.128 this now only gives us 128 addresses.

Let’s make it a /26 just take away 2 bits from the hosts and now you have 192.168.1.0 255.255.255.192 this now only gives us 64 addresses.

If we decided to use 2 /26 subnets from the top example we can continue onto the next subnet of 192.168.1.65 255.255.255.192.

 

The best thing you can do is play with subnets and start carving out your own on paper. Once you do that you’ll be a master in no time, just remember the above chart.