Super subnetting 2

[reddevil2]

Hi there
I understand the concept behind subnetting and finding the first and last sub number. But I have reached the point of study where super subnetting is needed. Can anyone help me with a short cut to working out this factor
Many thanks in advance

Reddevil2

 

[burtsbees]

Supernetting, you mean? Address summarization? For example...
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
192.168.5.0/24
So, in binary, convert the third octet into binary for /24...
0 0 0 0 0 0 0 1
0 0 0 0 0 0 1 0
0 0 0 0 0 0 1 1
0 0 0 0 0 1 0 0
0 0 0 0 0 1 0 1
Like that.
The, look at what bit position the bits match for all subnets...in this case, it is the fourth bit position. That is the way the third octet for the summarized address will look like...in this case, the first three bits are not common in all 5 subnets, and the last 5 are. So the summary address changes all the bits that do match to binary one, like this...
1 1 1 1 1
1 1 1 1 1
1 1 1 1 1
1 1 1 1 1
1 1 1 1 1
The summary address, or supernet, is 192.168.0.0/21

Here is more on that...

http://www.sethenol.com/supernetex.htm

Burt

 

[reddevil2]

Thanks Burt,

Does this theory also work with splitting subnet numbers such as 172.16.31.0/20 to 172.16.32.0/26. See I don't understand how you go from /20 to /26. Through the different subnets.

 

[CiscoGuy33]

reddevil2,

That would be VLSM or subnetting a subnet

Reddevil2, go to this website and you can download Chapter 3: IP Subnetting and Variable Length Subnet Masks (VLSMs)
FREE from the Sybex CCNA Study Guide (5th ed) by Todd L. - I think this is one of the BEST chapters ever written on subnetting

http://www.bookpool.com/sm/0782143911

He has a good explaination of this!

Hope this helps!!


E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[burtsbees]

I can't tell if you're confusing supernetting with vlsm, like CiscoGuy33 says...supernetting is for address summarization, so that you could write one acl entry to cover many closely related subnets, or for a router that can have one summarized route in its routing table, rather than 8, being more efficient.

Burt