6 Replies Latest reply: Jan 30, 2011 6:06 AM by Fabri

# QUICKLY CALCULATE THE SUMMARY ROUTE

hello guys,
I want to learn how to calculate the summary route quickly, without having to convert to binary IP address.
if not mistaken it is estimated multiple contiguous networks of 4 ... but I did not understand ... can anyone help me?

• ###### 1. Re: QUICKLY CALCULATE THE SUMMARY ROUTE

search for a post by John

he had posted some examplses with explanations of decimal way to summerize

• ###### 3. Re: QUICKLY CALCULATE THE SUMMARY ROUTE

Aloha Fabri,

There is no trick to doing this quickly unless you have already memorized certain prefix lengths and subnet mask relationships.

Also, this depends on the networks you wish to summarize.  For example, summarize the following three Class C addresses?

192.168.0.0/24

192.168.2.0/24

192.168.3.0/24

Note this summary address includes the 192.168.1.0/24 network which may or may not belong to you.

summarize the following 10 network?

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24

172.16.5.0/24

172.16.6.0/24

172.16.7.0/24

172.16.8.0/24

172.16.9.0/24

172.16.10.0/24

172.16.11.0/24

Note this summary address covers 172.16.0.0 to 172.16.15.255 and includes address not identified or that may not belong to you.  A an alternate summarization you could do the following:

172.16.2.0/23   --> cover 172.16.2.0/24 and 172.16.3.0/24

172.16.4.0/22   --> covers 172.16.4.0/24 to 172.16.7.0/24

172.16.8.0/22   --> covers 172.16.8.0/24 to 172.16.11.0/24

With the second summary example you do not include addresses that are not shown.  But either way is correct summarization.

Here is a little something that may help you in summarizing (supernetting)

/24 = 1 network

/23 = 2 networks

/22 = 4 networks

/21 = 8 networks

/20 = 16 networks

/19 = 32 networks

/18 = 64 networks

/17 = 128 networks

/16 = 256 networks.

this of course only applies to networks with a /24 prefix or subnet mask of 255.255.255.0.  But hopefully this will help you.

HTH

• ###### 4. Re: QUICKLY CALCULATE THE SUMMARY ROUTE

Nice find Martin.  Thanks John, very good reading.

• ###### 5. Re: QUICKLY CALCULATE THE SUMMARY ROUTE

Hi Fabri

You need to learn your sixteen times table and then visualize the subnets.

So say you had 10.0.3.0   and 10.0.17.0     then you can see it falls either side of 16 but not 32.

If you learn your 16 times table then you learn your 32 times up to 8 which is all you need.

so the summary is 10.0.0. to 10.0.64.0 - 1

so 64 is 2 to the six so the summary is 10.0.0.0/(24-6) or 10.0.0.0/18.

Regards Conwyn

• ###### 6. Re: QUICKLY CALCULATE THE SUMMARY ROUTE
Thank you very much for responding quickly and effectively to my question.
Now everything is clearer.
I also wrote a mini guide to remind Italian better in future.

Greetings!