How to Find Subnets

An Internet modem

A subnet mask, or subnet, is a string of numbers that your computer uses to browse the Internet along with other Internet protocol (IP) information like IP addresses, domain name servers and default gateways. When your modem attempts to connect to your Internet service provider's (ISP) network, it will obtain all IP info automatically and set up the connection for you. If you need to determine what subnet your computer is currently receiving from the modem, there are a few simple ways to do so.

Things You'll Need

• Active home internet connection
• Home computer running Windows (XP, Vista, or 7) or Mac OSX

Instructions

1. Bypass any network devices. To see what subnet your ISP is providing to your modem, you first need to bypass any network devices such as routers, switches or hubs that go between the modem and the computer. Locate the ethernet port on the back of your modem; it will resemble a phone jack, only slightly larger. Trace the other end of the cable. If it is plugged into anything other than your home computer, unplug it from whatever it is plugged into and plug it directly into one of the ethernet ports on your computer. You should now have a direct ethernet connection from your modem to your computer.

2. Reset the modem. When you move a broadband modem from one device to another, most modems will not recognize the new device until the modem is reset. This is done easily by unplugging the power supply from the modem for 30 seconds, plugging it back in, then allowing an additional 30 seconds for the modem to run through its boot cycle. The modem should now establish a connection with the computer.

3. Determine the subnet. Once the modem has fully synced up with your ISP's network, the modem should have a subnet mask. For Windows machines (Vista, XP or 7), click "Start" > "All programs" or "Programs" > "Accessories" > "Command prompt." In the black and white DOS screen that appears, type the command "Ipconfig." Among the command output, you will find a string of numbers listed next to "Subnet mask." For Mac machines, navigate to the "Network" icon on the "System Preferences" tray. Click on the line that reads "Built-In Ethernet," to highlight it, and click the "Configure" button towards the bottom. Your subnet mask will be listed along with all other IP information.

Tips & Warnings

• If your subnet mask is not listed, you may need to renew the connection to force the modem to resync with your ISP's network. For Windows machines, in the same command prompt window, type "Ipconfig/release" and the "Enter" key, then "Ipconfig/renew" and the "Enter" key. For Mac machines, on the Configure network screen, click the "Renew DHCP lease" button. If the subnet still is not listed after performing these steps, it is necessary to contact the technical support line of your ISP to check for a possible connection problem.