Simplified Subnetting

 

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

# of Bits (b)

# of Networks (2b)

1

2

2

4

3

8

4

16

5

32

6

64

7

128

*Please keep in mind that this chart applies to devices capable of using all 1s and all 0s for the network portion. Some devices may not support this so you would need to modify column 2 to 2b-2.

 

Use the above table to you find the number in column 2 that is equal to, or just greater than, the number of segments you require. Notice that the number of Networks is equal to 2 raised to the number of bits used for subnetting.

 

 

Example

For example, suppose you want need to subnet the network 201.168.55.0 into 4 segments. You would find the number 4 in the column labeled # of Networks and discover you need to subnet on 2 bits. This really means borrowing 2 bits from the bits normally reserved for the host portion of the address.

 

Since 201.168.55.0 is a Class C address with a Standard Subnet Mask of 255.255.255.0 we know we will be modifying the 4th octet (the range normally reserved for hosts) and using the first 2 bits for our subnet mask.

 

In binary, our 4th octet becomes 11000000 which is equivalent to 192 decimal. Therefore our subnet mask is 255.255.255.192.

 

Defining Network IDs

Once you’ve determined what subnet mask to use you now need to know which Network IDs to use (as well as Host Ranges). One easy way is this:

  1. Subtract your Subnet Mask from 256 (alternatively, divide 256 by the number of segments required)
  2. Starting with 0 increment by the number you computed in Step 1– these will become your network IDs.

 

For example, here are the segments derived from the original network 201.168.55.0 using a Subnet Mask of 255.255.255.192.

 

The networks are incremented by a value of 64 (256-192=64 and 256/4=64) See Step 1 above.

 

Defining Hosts and Broadcast Ranges

Network

Host Range

Broadcast Address

201.168.55.0

201.168.55.1 –

201.168.55.62

201.168.55.63

201.168.55.64

201.168.55.65 –

201.168.55.126

201.168.55.127

201.168.55.128

201.168.55.129 –

201.168.55.190

201.168.55.191

201.168.55.192

201.168.55.193 –

201.168.55.254

201.168.55.255

 

 

The Broadcast Address is always one less than the next Network ID while the Host Ranges is everything between the Network ID and the Broadcast Address. So if our Network IDs are incremented by 64, then we have 64-2=62 host addresses per subnet.