Operating systems, such as Windows NT, that support multiple IP addresses let you configure multiple virtual domains: one machine hosting several domains, or multi-homing. For example, two domains such as www.abc.com and www.xyz.com can reside on the same machine and serve independent information.

Setting up virtual domains in NT is simple and straightforward. Consider two domains with IP addresses www.abc.comĀ­204.55.157.20 and www.xyz.comĀ­204.55.157.21.

For these two virtual domains to run under NT, you need to add their IP addresses to the network settings: Go to Main, Control Panel, Network, and choose TCP/IP Protocol. Go to the Advanced section of the TCP/IP configuration. In the IP Address field, type in the first IP address (204.55.157.20). Move to the Subnet Mask field, and type in 255.255.255.0. Click Add, and click OK at the TCP/IP Configuration dialog. Click OK at the Network Settings dialog, and click Restart. When you see the prompt, repeat the same sequence for the second IP address.

If you ping either IP address from an Internet host, you now get a response. You've finished setting up the virtual domains on the NT server.

Now go to your Domain Name System (DNS) server, and add the entries for the two domains as if they were on different machines. This method lets you add up to five IP addresses to an NT machine. To add more than five, you need to edit the Registry. Warning: Using the Registry Editor incorrectly can cause serious, systemwide problems. You may have to reinstall NT to correct them, so use this tool at your own risk.

To add an IP address through the Registry Editor, go to the command prompt and run regedt32.exe and find the key hkey_local_machine\system\currentcontrolset\services. Scroll down until you find your machine's network card. Click Parameters, and then click Tcpip.

At this point, you see the Registry Editor menu with TCP/IP parameters on the right side of the window. From the TCP/IP parameter list, double-click IP Address.

The Multi-String Editor dialog will open to display the current IP addresses, as in Screen A. To add more IP addresses, enter them here, and click OK when you finish. Repeat this process for each SubnetMask. Restart your computer so these changes take effect.

For a Web server to support multiple virtual domains on one machine, the Web server software must support virtual domains. Alibaba (version 2.0) by Computer Software Manufaktur of Austria is one the few NT Web server packages that support virtual domains. (For a review of Alibaba, see Joel Sloss, et al., "Web Server Software Roundup," page 57.) To use software such as Alibaba, create a directory for each virtual domain on the Web server. Suppose you create the directories E:\abc and E:\xyz for the domains abc.com and xyz.com; all information and Web pages for company abc.com will reside in E:\abc and its subdirectories, and all information and Web pages for company xyz.com will reside in E:\xyz and its subdirectories. To make abc.com's Web pages available for browsing over the Internet, open the Alibaba Administration Tool, go to the Document Root, enter the IP address (204.055.157.020) for abc.com in the IP Address field, and enter the root directory (E:\abc) for abc.com in the Server Root field. Click Add and OK in the Alibaba Administration dialog. Repeat the sequence for xyz.com.

Now users can browse both www.abc.com and www.xyz.com without knowing whether the two URLs are on one machine or two. If your Web server is also an email server that supports virtual domains, you can send email to users in each domain. For example, Software.com's Post.Office (for a review of Post.Office, see Jeffrey Sloman, "SMTP Mail Servers for NT," July 1996) lets you add domain names to an email account through form-submission. You add the account form Internet Access address such as user_name@www.abc.com or user_name@www.xyz.com.