Subnetting Techniques

Procedure for implementing Subnets:

• Determine the IP address that is assigned by the registry authority
• Based on the organizational and administrative structure, determine the number of subnets that are required
• Depending on the address class and required number of subnets, decide the number of bits that you need to steal from the Host ID
• Evaluate the binary and decimal value of the subnet mask
• Apply the subnet mask to the network IP address to determine the subnet and host addresses

Pretty cumbersome, I suppose !!!!  Fortunately there is an easier method as you can see below:

Short cut method:

Note that the Subnet mask is /29.  That would mean we have stolen five bits from the default mask of a Class C /24 address.

Go five columns deep in the below table, as shown:

The number that you get when you use this method is called the Magic Number.  It is 8 in our example.

Your subnets increment on a factor of 8 i.e.
0     - 7
8     - 15
16   - 23
24   - 31
32   - 39
40   - 47
48   - 55
56   - 63
64   - 71
........
........
........
224 - 231
232 - 239
240 - 247
248 - 255

That's 32 networks with 8 IPs for each subnet.  

Our IP 192.168.221.37 falls in the 32 - 39 range.  So the evaluation goes as:

Use the following table as an aid while calculating subnets: