Determine Subnet Name
Here, we'll apply these rules for determining a subnet name to a specific example so that you can see how the calculations are done.
1.) The first step in determining the name for your new subnet is to identify the IP addresses and subnet mask that computers on this subnet will use. For example, you want to set up a site containing a computer with an IP address of 172.16.10.200 and a subnet mask of 255.255.255.0. Begin by converting your subnet mask from the dotted decimal notation to a binary value. You can do this in Microsoft Calculator by switching to the Scientific view, entering each octet, and then switching from decimal (Dec) display to binary (Bin) display.
The decimal formatted numbers of the example, 255.255.255.0, convert as follows:
| Decimal value |
Binary value |
| 255 |
11111111 |
| 255 |
11111111 |
| 255 |
11111111 |
| 0 |
00000000 |
Note: Remember that in dotted decimal notation, each octet
represents eight bits, so the zero in the last octet of the subnet mask is actually represented by eight zeroes in binary notation.
Combining these binary numbers yields a subnet mask of 11111111 11111111 11111111 00000000 in binary notation. In the binary representation of a subnet mask, ones indicate the network or subnet ID portion of the address. The first 24 bits in this address will be read as the network ID because those bits have ones in the subnet mask. This value will be used for the bits masked portion of the subnet name.
2.) The next step is to determine the network ID value in the subnet name.
The IP address in the example, 172.16.10.200, converts as follows:
| Decimal value | Binary value |
| 172 | 10101100 |
| 16 | 00010000 |
| 10 | 00001010 |
| 200 | 11001000 |
Combining these binary numbers yields an IP address of 10101100 00010000 00001010 11001000 in binary notation. Now follow these four steps:
- Enter this binary number without the spaces between the octets into Calculator.
- Press the AND button.
- Enter the subnet mask in binary notation.
- Press the equal button (=).
The resulting number in binary notation is 10101100 00010000 00001010 0000000. This binary number converts to a decimal value as follows:
| Binary value | Decimal value |
| 10101100 | 172 |
| 00010000 | 16 |
| 00001010 | 10 |
| 00000000 | 0 |
Combining these decimal numbers yields a network number of 172.16.10.0.
The subnet name for this example is 172.16.10.0/24.
And you are done.