How to Use Multiple Subnets

In TCP/IP networking, a “subnet” is a portion of the network that has been closed off from making connections to other computers on the same physical network. This can be used to improve local network responsiveness, or to specifically route traffic over particular network ports when using a computer as your router.

Instructions

1. Set up the first computer to act as the router for your first subnet. Local IP addresses are restricted to ranges that have been set aside for this purpose; the most commonly used are 10.x.x.x, and 192.168.x.x. Select the Ethernet port, and manually assign it these TCP/IP networking values:

   Network address: 10.0.10.1
   Subnet mask: 255.255.255.0
   Router address: 10.0.10.1

2. Network additional computers to this subnet by selecting their Ethernet ports, and entering these values:

   Network address: 10.0.10.2 (increase the “2” by 1 for each additional computer)
   Subnet mask: 255.255.255.0
   Router address: 10.0.10.1 (must be the same router address for all computers)

   This creates the first subnet, over the Ethernet connection.

3. Return to the original routing computer to create the second subnet. Select the wireless port, and manually assign it these TCP/IP networking values:

   Network address: 10.0.1.1 (note that the third value is “1,” not “10”)
   Subnet mask: 255.255.255.0
   Router address: 10.0.1.1

4. Repeat Step 2 for the computers you wish to connect wirelessly to the router computer, using these values:

   Network address: 10.0.1.2 (increase the “2” by 1 for each additional computer)
   Subnet mask: 255.255.255.0
   Router address: 10.0.1.1 (must be the same router address for all computers)

   This creates the second subnet.

5. Choose any of the computers that are networked over Ethernet. Open the ping network utility at a Macintosh, DOS or Unix command line prompt, and connect to the router computer at the address 10.0.10.1 (i.e., “ping 10.0.10.1”). You will see a successful connection to the router computer. You will also be able to ping any other computer connected via Ethernet to this subnet.

6. Attempt to ping one of the wireless computers from the same Ethernet-networked computer, i.e., “ping 10.0.1.2”. This will fail, because the subnet mask blocks computers on the 10.0.10.x network from seeing computers on the 10.0.1.x network. Only the router computer, which is on both subnets, can communicate with all computers on either subnet.

7. Create a new network connection on the same Ethernet-networked computer. Select the wireless port, and enter the following values:

   Network address: 10.0.1.x (replace "x" with the number of wireless computers plus 1; it must be unique)
   Subnet mask: 255.255.255.0
   Router address: 10.0.1.1

   This computer will now be on both subnets. Retry the “ping 10.0.1.2” ping command; it will now work.

8. Return to the router computer, which is now connected to the computer from Step 7 over two subnets simultaneously. Transfer a large file to that computer over the wireless network, with an application such as FTP; i.e., “ftp 10.0.1.x” (use the "x" from Step 7). The “1” in the address “10.0.1.x” uniquely specifies the wireless network. The file will transfer at wireless speeds--which will mean a long wait for a large file. Cancel the transfer.

9. Connect to the same computer from the router computer, but this time use the Ethernet network; i.e., “ftp 10.0.10.x” (use the "x" from Step 4, which you assigned to that computer). This time, the file will transfer over the Ethernet cable--and will be approximately 10 to 100 times faster.

10. Add additional subnets as desired. You can only assign a single IP address to a wireless port, but Ethernet will support multiple subnets on a single physical port; refer to your operating system help files for instructions on how to create additional “logical” ports on a single physical port.

Tips & Warnings

• Many local area networking protocols work on a “broadcast” method, which can congest a large network that has not been subnetted. Subnets block computers from seeing each other, reducing broadcast traffic and increasing efficiency.

• Use subnetting addresses that serve as mnemonics. For example:

• 10.2.10.x: computers on the second floor, on Ethernet (which used to use “10”-baseT)

• 10.3.5.x: computers on the third floor, connected over FireWire/IEEE 1384 (5 for FireWire)

• 10.0.1.x: computers anywhere, connected over wireless (“1” because it’s frequently the most convenient network)