Chapter 30: Connecting Your LAN to the Internet

What Is Internet Connection Sharing?

Windows XP's Internet Connection Sharing (or ICS) is a NAT program, and doesn't provide caching or logging. ICS allows one computer--the ICS server--to provide an Internet connection for all the computers on a LAN. The ICS host runs the ICS server program. The other computers--the ICS clients--on the LAN can run Windows Me, 9x, 2000, NT, older Windows versions, or other operating systems, as long as they support TCP/IP.

ICS uses private IP addresses in the format 192.168.0.xxx. You can't use static IP addressing (Microsoft claims that there's a way, but we haven't had any luck). ICS includes a DHCP (Dynamic Host Configuration Protocol) server, which runs on the ICS server and assigns IP addresses to the rest of the computers on your LAN automatically. DHCP assigns the address 192.168.0.1 to the ICS server itself.

Figure 30-1 shows a LAN with five computers, including the ICS server computer, with private IP addresses from 192.168.0.1 to 192.168.0.5. The ICS server has a separate IP address for communicating with the Internet over a DSL line; this address is assigned by the ISP (in the figure, it has the address 245.62.168.33).

Figure 30-1: A LAN with Internet Connection Sharing

ICS includes these components:

• DHCP Allocator Assigns IP addresses to ICS client computers on the LAN
• DNS Proxy Translates between IP addresses and Internet host names (like www.yahoo.com), using your ISP's DNS server
• Network Address Translation (NAT) When passing packets of information between the LAN and the Internet, replaces the private IP address with the ICS server's IP address, and vice versa

The next section describes how to install ICS on the ICS server and how to configure the rest of the computers on the LAN to share the connection.

Windows also comes with an Internet Connection Firewall to provide security for any Internet connection, whether shared or not.