Summarization tricks and tips?

mark076h

I mastered regular subnetting from the guide here. Now does anyone know a quick way to come up with a summary network. Say you are given a question with 4 networks you want to summarize, instead of doing out the binary for each one to determine the common bits is there some kind of simple subtraction you can perform to speed it up?

So for example-

Summarise the following:

192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24

how would you get the 192.168.3.0/XX mask easily without doing out the binary?

MickQ

4 subnets, starting from 0.

2^x = 4 x=2

/24 - 2 = /22


192.168.0.0 /22

NetworkVeteran

mark076h wrote: »

Summarise the following:

192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24

"Doing out the binary" in this case can be done in one's head in seconds. 192.168.000000xx.xxxxxxxx = 192.168.0.0/22. I can't remember ever needing to subnet in less than 2-5 seconds.

Ltat42a

Check out this tip -
Subnetting Made Easy And Other Cisco Tidbits: Route Summarization

Zartanasaurus

NetworkVeteran wrote: »

"Doing out the binary" in this case can be done in one's head in seconds. 192.168.000000xx.xxxxxxxx = 192.168.0.0/22. I can't remember ever needing to subnet in less than 2-5 seconds.

"If he doesn't summarize those prefixes in under 5 seconds a bus full of senior citizens will explode!!!"

I dunno if I have any kind of tricks or tips for this kind of thing other than practice/experience. I looked at that and right away knew that it was 252 for the 3rd octet. 192.168.0.0 starts on a boundary and the number of addresses you have to summarize is a power of 2, so it works out very cleanly. Those will be very easy to do in your head with a little bit of practice.

256 - (Lowest power of 2 >= total of addresses to be summarized) will take care of your "easy" scenarios when all of the subnets fit cleanly in the range IE 0-3, 4-7, 8-11 or 0-15, 16-31, 32-47 etc etc and they aren't asking you to not summarize networks that you don't actually have.

mella060

Well by the time you get to route summarization, you should be comfortable with binary, so you should be able to look at numbers and work out their binary values in your head. In your example you have...

192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24

So you know that 1 in binary is 00000001
2 in binary is 00000010
3 in binary is 00000011

Now with route summarisation, the 1st step is to list all the subnets to be summarized in binary. Again, if you are comforttable with binary then just visualize it in your head. Look at the octet where the values change. In your example, the values change in the 3rd octet. The first two octets are the same...192.168

In the 3rd octet, they change from 0 to 3

Anyway just to write out the subnets in binary

192.168.0.0/24 - 11111111.11111111.00000000.00000000
192.168.1.0/24 - 11111111.11111111.00000001.00000000
192.168.2.0/24 - 11111111.11111111.00000010.00000000
192.168.3.0/24 - 11111111.11111111.00000011.00000000

So then from left to right, determine the "in-commom" bits where all the values are the same. In the example, the first 22 bits are the same.

To find the summary routes subnet number, change all values to the right of the in-common bits to zero. So the last 10 bits

The subnet number is 192.168.0.0 /22