Adapted by Beau Sanders from an article originally published on the Internet by Greg Stanley, MCP

Let’s begin by creating the following chart based on the formula 2^N -2 = X:

First, write down a binary 1 for every bit in an octet:

1 1 1 1 1 1 1 1

Next, ABOVE each bit, write its value:

128 64 32 16 8 4 2 1

1 1 1 1 1 1 1 1

Now, BELOW each bit, write the value of that bit plus the other bits to its left:

128 64 32 16 8 4 2 1

1 1 1 1 1 1 1 1

128 192 224 240 248 252 254 255

Finally, BELOW all that, write the value of 2^N-2, starting from the left. This will give you the number of subnets:

128 64 32 16 8 4 2 1

1 1 1 1 1 1 1 1

128 192 224 240 248 252 254 255

0 2 6 14 30 62 126 254

Now lets see how this all works. In our first example we are given the Network ID: 192.62.123.0

What subnet mask will you need for 10 subnets?

A quick look at the chart indicates that 224 will give you 6 subnets, but 240 yields 14 subnets. We need 10 subnets which is less than 14 and more than 6. The answer: 255.255.255.240

All bits to the right of 240 (including other octets if Class A or B) make up the Host ID. The 240 and everything to the left of that compose the Network ID.

Now let’s look at the ranges of subnets, also called increments of subnets. Look above 240 and you see 16. This is the first subnet address, as well as the range incrementing value. The next subnet address will be 16 + 16, or 32. The next will be 32 + 16, or 48, and so on. The first address in each range is the address of the subnet itself (the Subnet Network ID) and the last address is the broadcast address for that subnet. The first and last addresses are not considered usable subnet addresses.

Easy Subnetting 20080111.doc Page 1 of 3 Easy Subnetting

The following are the ranges (increments) for our Class C network in this example:

Broadcast Address    Last Host Address    First Host Address   Subnet Address   

0	192.62.123.0	192.62.123.1	192.62.123.14	192.62.123.15
1	192.62.123.16	192.62.123.17	192.62.123.30	192.62.123.31
2	192.62.123.32	192.62.123.33	192.62.123.46	192.62.123.47
3	192.62.123.48	192.62.123.49	192.62.123.62	192.62.123.63
4	192.62.123.64	192.62.123.65	192.62.123.78	192.62.123.79
5	192.62.123.80	192.62.123.81	192.62.123.94	192.62.123.95
6	192.62.123.96	192.62.123.97	192.62.123.110	192.62.123.111
7	192.62.123.112	192.62.123.113	192.62.123.126	192.62.123.127
8	192.62.123.128	192.62.123.129	192.62.123.142	192.62.123.143
9	192.62.123.144	192.62.123.145	192.62.123.158	192.62.123.159
10	192.62.123.160	192.62.123.161	192.62.123.175	192.62.123.175
11	192.62.123.176	192.62.123.177	192.62.123.190	192.62.123.191
12	192.62.123.192	192.62.123.193	192.62.123.206	192.62.123.207
13	192.62.123.208	192.62.123.209	192.62.123.222	192.62.123.223
14	192.62.123.224	192.62.123.225	192.62.123.238	192.62.123.239
15	192.62.123.240	192.62.123.241	192.62.123.254	192.62.123.255