Supernetting or Classless Inter Domain Routing in Computer Networks

Supernetting or Classless Inter Domain Routing in Computer Networks

Supernetting  or Classless Inter Domain Routing.

Supernetting is the process of aggregating or combing two or more networks. IPv4 support various classes namely CLASS A, CLASS B, CLASS C and CLASS D.
 
We can only combine classes of same type . That means we can combine two or more CLASS C network together. But we cannot combine a CLASS B network and a CLASS C network together to form a single network. 
 
For example we can combine two class  C  Networks.But it is not possible to combine a class B network and a class C network.Supernetting is also known as CIDR (classless inter domain routing).
 
  • Supernetting
  • Classless Inter Domain Routing
  • Combine multiple Network id's to a single network
  • To reduce the wastage of IP Address
 
Supernetting is just opposite to subnetting. That means in Subnetting  bits are borrowed from the host part to form the subnet id. But in Supernetting bits are borrowed from network part and the borrowed bits are made part of the host part.

Supernetting Procedure steps

1. Identify / List  the Networks to combine.

2. Ensure that all networks are of the SAME CLASS.

3. Convert all  IP addresses to corresponding binary format.

4. Find the common prefix from them in the binary format. Common prefix means the set of matching bit sequence from left to right.

5. Set the remaining bits to zero.

 

The number of bits in the common prefix will be less than the umber of bits in the network part of the given IPs.

 

Example:

192.168.98.0     11000000 10101000 01100010 00000000

192.168.100.0   11000000 10101000 01100100 00000000

192.168.105.0   11000000 10101000 01101010 00000000

--------------------------------------------------------------------------------------------

The part represented in red colour represet the common prefix. that means this part is same for all . 

Summarized route or Common Address= 11000000 10101000 01100000 00000000 

                                                         = 192.168.96.0 / 20

Length of the common prefix = Length of Network part = Netmask = 20

Thus subnet mask contains 20 ones followed by 12 zeros. = 11111111 11111111 11110000 00000000

                                                                                            = 255.255.240.0

 

 Real Time Situations when  Supernetting is Useful

1. An organization need to connect 300 computers in a network

Consider the following requirement. An organization need to connect 300 computers in a network.
 
A class C network can accommodate maximum of 28 - 2 = 254 hosts.
 
So we have to select a Class B network to full fill this requirement and can accommodate a maximum of 216 - 2 = 65534 hosts.
 
But if we use a class B network for this there is a wastage of  65534 - 300 = 65234 IP addresses.
 
Here comes the importance of supernetting. We can combine two class C networks and it can accommodate all 300 hosts.The  maximum  hosts possible on this network is 2 *  254 = 508 hosts. Here the wastage of IP addresses is considerably reduced.Here the wastage is only 308 IP addresses.
 
2. An organization need to connect 1000 computers in a network
 
A class C network can accommodate maximum of 28 - 2 = 254 hosts.
 
So we have to select a Class B network to full fill this requirement and can accommodate a maximum of 216 - 2 = 65534 hosts.
 
But if we use a class B network for this there is a wastage of  65534 - 1000 = 64534 IP addresses. 
 
We can combine Four class C networks and it can accommodate all 1000 hosts. The  maximum  hosts possible on this network is 4 *  254 = 1016 hosts. Here the wastage of IP addresses is considerably reduced.Here the wastage is only 16 IP addresses.
 

 Supernetting Advantages

Thus by combing class C Address space we can prevent the wastage or exhaustation of reserved Class B Address space. Hence the conservation of  reserved IP Address space is one of the biggest advantage of supernetting.  Also it helps to reduce the number of entries in the routing table.