Using DHCP with Internet Connection Sharing

The most popular network protocol in use today, TCP/IP, requires that each computer on a network have a unique number called an IP address. If you have two network adapters in your computer, each must have a different IP address. For example, when you connect to the Internet, your DSL or cable modem will have a different IP address than the network adapter connected to your network.

IP numbers are hierarchical in nature. A number called a subnet mask is used in conjunction with the IP address to enable the segmentation of a large network into smaller networks. What's important for you to remember is the specific set of numbers used in your home network.

There are groups of IP addresses that are specifically reserved for small networks. One of the groups of IP addresses is used by Internet Connection Sharing (ICS)—192.168.x.1 and 192.168.x.254 where x represents a number between 0 and 255.

These numbers are used in conjunction with a subnet mask of 255.255.255.0. This particular subnet mask is known as a Class C subnet.

Understanding the IP addressing scheme and using it appropriately can be difficult. ICS automates the IP numbering task for the ICS clients on your network with the Dynamic Host Configuration Protocol (DHCP) server service. The DHCP service enables the ICS host computer to assign IP addresses to its clients automatically. By default, when ICS is installed, the DHCP service begins supplying addresses to computers on the network.

Your Internet service provider (ISP) may use a DHCP service to assign your computer an IP address when you connect to the Internet. This is commonly referred to as a dynamic IP address. Each time your computer connects to the Internet, a different, but unique number is assigned to it.

When you install ICS, assuming you have two adapters in the computer—one for the device connecting you to the Internet and another for connecting to your home network—an IP address of 192.168.0.1 is assigned to the network adapter connected to your network. This is assigned permanently and is referred to as a static IP because it does not change. Next, the DHCP service is loaded into memory so that IP addresses can be assigned to computers on the network.

Each client computer must request an address. It is important that the ICS host computer remain on all the time or be started prior to starting the other computers on your network. Otherwise, the client computers will be unable to obtain an IP address.

If you do not want to use the DHCP server service you can set up your clients with static IP addresses. Assuming you have only two adapters in the ICS host computer, you may set up the network in the following manner:

• Do not change the configuration of the ICS host computer
• Assign each client computer a unique IP address in the 192.168.0.2 - 192.168.0.254 range.
• Assign each client computer a subnet mask of 255.255.255.0
• Enable DNS on each client computer and enter a unique host name—the name of the client computer as it appears on the identification tab of Network properties is recommended. Under DNS Server Search Order add 192.168.0.1.
• Enter a Default Gateway of 192.168.0.1 on each client computer.

If you have more than two adapters in the ICS host computer, multiple subnets are required to make your network function properly and share Internet access. ICS automatically configures the host computer.

Notes

• Internet Connection Sharing does not work with some versions of AOL. Please contact AOL for more information.
• Your ISP may charge you for connecting multiple computers through a single Internet connection. Check with your ISP for details.