How to Calculate Subnet From an IP

A subnet is sometimes also called a subnet mask. It is written in the same format as an IP address, which consists of four numbers separated by periods. Each section of numbers can be zero through 255, which represents eight binary digits. The subnet determines the amount of IP addresses that can be used within a network. They are fairly lenient and can be set to include more IPs than needed, because IP addresses are virtual.

Instructions

1. Open the "Start" menu. Go to the "Accessories" folder under the "Programs" menu. Click on the "Command Prompt" to open it.

2. Type in "ipconfig" then press the "Enter" key. This will display your computer's current IP address and default gateway. An example would be an IP of 172.16.0.1 and a default gateway of 172.16.1.64.

3. Count the number of sections in the IP and default gateway that match starting from left to right.

4. For each section that matches the subnet will be 255. The rest of the sections can be zero. For the example listed above, a subnet of 255.255.0.0 would work.

Tips & Warnings

• Most computers use a subnet of 255.255.255.0.

References

• Network Clue: Calculating Subnet Masks
• Dialogic: Calculating a Subnet Mask
• Josh Gentry: Classful IP Subnet Calculations