05 Mar 2011 - Lab-Rat
In the last note we talked about subnetting. Sometimes instead of breaking network into smaller chunks, we have to go the other way and summarize. To do this quickly in decimal, we have to do a bit of work using powers of 2. It’s pretty similar to the old CCNA days when you needed to figure out how many hosts or networks were available in each subnet mask.
1. List your subnets (the networks, not hosts, see the subnetting post for a shortcut on finding the network).
2. Count the number of networks, and figure out the closest power of two.
3. Subtract that number from the mask.
4. Check and make sure all your networks fall within the range.
5. If they do, you’re done.
6. If not, subtract one from the mask bit and repeat step 4.
Let’s do an example:
172.16.31.0/24
172.16.28.0/24
Okay, two networks, one bit borrowed from the mask gives us two networks- so let’s try that.
172.16.31.0/23 is contained in the 172.16.30.0 network.
172.16.28.0/23 is contained in the 172.16.28.0 network.
(If this isn’t intuitive yet, check out the subnetting post again.)
Humn, they don’t match- let’s take another mask bit away and try again.
172.16.31.0/22 is on the 172.16.28.0 network.
172.16.28.0/22 is on the 172.16.28.0 network.
Bullseye, there’s our summary.
Additional Reading:
CCIE Routing and Switching Certification Guide