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| posted in Cisco Networking, Technical | |
| by Steve on June 14th, 2010 | tags: binary, summary route |
As your network gets larger or you move on to administer a larger network, you will most likely find yourself wanting to summarize your routing table. This article covers the methodology for determining the perfect summary route for your list of subnets.
Why a ‘perfect’ summary and not just a summary? It’s pretty easy to determine a huge summary route for any given list of subnets, but that’s not desirable. If you summarize 4 /24s to a /16 or /8 instead of a /22 or /21, you may get packets crossing your network with no real destination. By finding the smallest possible summary route for your list of subnets, you will reduce this traffic and have a much ‘cleaner’ routing table for future expansion.
So let’s work through an example using these subnets:
10.1.1.0/24 10.1.2.0/23 10.1.4.0/21 10.1.9.0/24
Convert the Subnets to Binary
Convert each octect from your list of subnets to their binary equivalent and list them.
00001010.0000001.00000001.00000000 00001010.0000001.00000010.00000000 00001010.0000001.00000100.00000000 00001010.0000001.00001001.00000000
Find the In-Common Bits
Starting from the left, look at each column of binary digits for all of your subnets until you find the first column that is have different values. All bits before this column are the ‘in-common’ bits.
00001010.0000001.00000001.00000000 00001010.0000001.00000010.00000000 00001010.0000001.00000100.00000000 00001010.0000001.00001001.00000000
Calculate the Summary Subnet Number
This is done by changing all of the not in-common bits in any one of your subnets to 0, combining those bits with your in-common bits, and then converting back to dotted decimal.
00001010.0000001.00000000.00000000
10.1.0.0
Calculate the Summary Subnet Mask
Now convert your in-common bits all to 1, combine that with the all 0s of the not in-common bits in the subnet number and then convert the result to dotted decimal.
11111111.11111111.11110000.00000000
255.255.240.0
The summary address for the example list of subnets above is 10.1.0.0 255.255.240.0.
Check Your Work
You can now check your work by calculating the host range of the summary subnet and mask and ensuring that that range covers all of the subnets from your list. Practice this by visiting the site’s Practice Subnetting page.
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