Route Summarization

cdad2000

Hey Team,

I hit a stump early on the BSCI. I can subnet well, but I'm drawing a blank on summarization.

Would you please breaking it down to me like as if I were a 4yr old. Thanks in advance. Go Giants!

dtlokee

You can find a few threads on this in the CCNA forum, but basically summarization is the process of reducing the number of bits in the subnet mask (the ones) to make multiple subnets look like one subent. I't basically the reverse operation of subnetting, if you reduce the mask by 1 bit it summarizes 2 subnets together (remember 2**n) if you reduce it by 2 bits it would summarize together 4 subnets. Now like subnetting, supernetting is also in even increments of whatever the 2**n value happens to be.

so

192.168.100.0/29
192.168.100.8/29
192.168.100.16/29
192.168.100.24/29

can all be summarized into 192.168.100.0/27 (if you figure out the subnet and broadcast address for 192.168.100.0/27 it would be 192.168.100.0 - 192.168.100.31 which is the same range as the 4 subents we're looking to summarize)

Now let's say you have a number of subnts that are different sizes and we want to summaize.

192.168.1.0/29
192.168.1.8/29
192.168.1.16/28
192.168.1.32/27
192.168.1.64/26

if you find the subnet address for the lowest subnet and the broadcast for the highest subnet you will find 192.168.1.0 to 192.168.1.127, which would be the same as a 192.168.1.0/25

Now in production, your subents may not work out so neatly and you may overlap some other subnets. This is where the idea of longest match routing helps.

cdad2000

Thank you for your help dtlokee.