tim.giles Tue, 10/07/2008 - 22:57

Hi,

Supernetting was created to try and solve the problem of routing tables growing beyond the ability of current software and people to manage. It provides a solution to the Class B network address spaces running out.

Supernetting allows one routing table entry to represent an aggregation of networks much like one telephone area code represents an aggregation of telephone numbers in an area.

Basically it combines several IP network addresses into one IP address. It reduces the number of entries in a routing table.

It's also called Classless Inter-Domain Routing (CIDR)..

..you'll need to know how it works for various exams including the CCNA.

Hope this helps.

Please rate useful posts...

Hi There

Supernetting is just another name for CIDR (Classless Inter Domain Routing) route aggregation. It's when you take a number of contigous subnets of the same class and aggregate then into a single network address or "Supernet".

i.e.

Say you have the following IP address ranges

172.16.0.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

Rather than have four separate entries in your routing table for these subnets you can aggregate them into a single network advertisement, the "Supernet" 172.16.0.0/22.

The /22 shows us that the first 22 bits of the IP address refer to the network portion of the IP address and the remaining 10 bits are used for host addressing.

So if the first 22 bits are used for the Network, the netmask will be 255.255.252.0 or in binary 11111111.11111111.11111100.00000000.

That means that there are 2 bits in the third octet (the last two 0's) usable for host IP addresses.

So the only possible values for the third octet are

00 = 0

01 = 1

10 = 2

11 = 3

Which are the values shown in the third octet in the IP ranges shown above. So this single /22 Supernet covers your four individual /24 subnets.

HTH

Best regards,

Michael

Richard Burts Wed, 10/08/2008 - 09:58

shafi

I like the way that Michael explains this. His explanation is in terms of aggregating subnets within a network. My experience of the term supernetting is frequently with aggregating addresses beyond the normal network boundary. So I would suggest that an example of supernetting would be to advertise:

192.168.0.0

192.168.1.0

192.168.2.0

192.168.3.0

as

192.168.0.0 255.255.252.0

so that you get 4 subnets (or networks) advertised in 1 address block, and that it crosses the normal network boundary.

HTH

Rick

shafistrokes Sat, 10/11/2008 - 23:11

hello Michael

thank you for your response

it is very useful to me

let me ask one more question related to the topic

is the route summerization is synonym to supernetting?

thanking you

shafi

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