Subnetting Example

Simplified Supernetting

An easier way to learn how to supernet is to create a table as follows:

# of Bits (b)	X=8-b	# of Networks (2^x)
1	7	128
2	6	64
3	5	32
4	4	16
5	3	8
6	2	4
7	1	2

Use the above table to you find the number in column 3 that is equal to, or just greater than, the number of networks you’re needing to combine. (Keep in mind, though, if you use the next largest increment, you could run into problems if your organization doesn’t own all the addresses in that range) Notice that the number of Networks is equal to 2^8-b where b is the number of bits used for supernetting.

Example 1

For example, suppose you want to supernet the 16 Class C networks

201.168.0.0 through 201.168.15.0 into a single Supernetted Address Range.

You would find the number 16 in the column labeled # of Networks and discover you need to supernet on 4 bits. This really means borrowing 2 bits from the bits normally reserved for the network portion of the address.

Since 201.168.0.0 is a Class C address with a Standard Subnet Mask of 255.255.255.0 we know we will be modifying the 3^rdoctet (the last range normally reserved for network addresses) and using the first 4 bits for our network mask.

In binary, our 3^rdoctet becomes 11110000 which is equivalent to 240 decimal. Therefore our subnet mask is 255.255.240.0.

If you look at the entire subnet mask in binary you can see how many addresses are reserved for the hosts:

11111111.11111111.11110000.00000000

nnnnnnnn.nnnnnnnn.nnnnHHHH.HHHHHHHH

All the positions where 0s or Hs are present represent the Host address portion. There are 12 H’s present, so 2¹²=4096 which is the same as 256*16(the number of networks we were originally trying to combine).

Example 2

What if we want to supernet the networks 201.168.10.0 through 201.168.17.0? According to our directions above you would look for 8 (the number of networks we’re trying to combine) in the # of Networks column resulting in using 5 bits for supernetting.

In binary, our netmask looks like this: 11111111.11111111.11111000.00000000 = 255.255.252.0

Remember how a router (or any TCP/IP enabled device) determines if something is on a local or remote subnet. It ANDs the destination IP address with its subnet mask. Below are the results of ANDing our 8 Class C networks with our computed Subnet Mask. Notice, what happens starting at subnet 201.168.10.16 (the result of the AND changes, indicating that 201.168.15.0 and 201.168.16.0 are on different subnets).

Network 1: 11001001.10101000.00001010.00000000 (201.168.10.0)