Way to Aggregation Subnet

kian_hong2000 · ‎02-25-2008

I have this question.

Say I have the these range of subnets:

172.18.4.128/25

172.18.4.0/25

172.18.5.0/24

172.18.6.0/24

172.18.7.0/24

And the closest super subnet is 172.18.4.0/22

1) I heard from my friend that the lowest number will usually be the super subnet number. Is it true?

2) What is the most fastest way to calculate the super subnet IP?

Jon Marshall · ‎02-25-2008

Hi

1) The supernet will be 172.18.4.0/22

2) The way i would work it out is

Subnet mask 22 = 255.255.252.0

256 - 252 = 4.

So your networks increase by 4 and will take you the next multiple of 4 ie.

172.18.0.0/22

172.18.4.0/22

172.18.8.0/22

etc..

Hope this makes sense

Jon

keeleym · ‎02-25-2008

Hi Kian

Just to add to what Jon has posted.

Don't get sucked into the "lowest number will be the supernet number" idea.

When you are given IP ranges like your example where they start and finish on a supernet boundry, then yes, the lowest subnet will be the supernet number.

However suppose you were given

172.18.5.0/24

172.18.6.0/24

172.18.7.0/25

172.18.7.128/25

172.18.8.0/23

172.18.10.0/24

172.18.11.0/24

172.18.12.0/24

In this instance the lowest subnet number does not fall cleanly on a supernet boundry and therefore would NOT be the supernet number.

Personally speaking I would summarise these subnets as follows

172.18.5.0/24

172.18.6.0/23

172.18.8.0/22

172.18.12.0/24

There by reducing the number of routes in a routing table from 8 down to 4.

HTH

Best Regards,

Michael