Why Multihome? There are many reasons to configure Windows NT as a multihomed server. You may want to host multiple virtual Web sites, each with its own IP address and host name, or gain additional fault tolerance and improve server performance.

The most common use of multihoming is to bind multiple IP addresses to a single NIC. This configuration is well-suited for Web servers since it lets a single server host multiple virtual Web sites. For example, an engineering department might want to host one Web site for development code and another site for production code on a single server. All commercial NT Web servers, such as those available from Microsoft Corp., Netscape Communications Corp. and O'Reilly and Associates, provide support for multihoming. Some Web servers, such as Nets cape's Enterprise Server, also allow for multiple virtual Web sites to have the same IP addr ess instead of using an IP address for each virtual Web server. The new HTTP 1.1 specification incorporates this feature.

While NT does support multiple IP addresses, with NT Server 3.51, you can enter only up to six IP addresses per network adapter via the GUI. If more are required, it may be necessary to edit the NT Registry directly--a potentially dangerous move. The registry is not needed with NT 4.0 Editing.

A Microsoft TechNote document states that a maximum of five IP addresses per network adapter is preferred. Based on our testing of NT 4.0 we've found that you can safely bind 15 addresses. We would not recommend more than 15, however; our testing of NT 4.0 becomes unstable beyond that number.

A second type of multihoming is where two or more NICs are installed in the same server and attached to the same network segment. This type of installation lets you have a different IP address/host name (and maybe a different virtual Web server) bound to each NIC.

This model also provides rudimen tary load-balancing and can provide fault tolerance, since you have multiple IP addresses (and thus multiple NICs) resolving to the same NetBIOS name. With the advent of hot-plug PCI from NetFRAME Systems and Compaq Computer Corp., if one of the PCI NICs does fail, you can easily remove the failed NIC and add a replacement without having to shut down the server. In addition, many NIC vendors, including Compaq and Madge Networks, provide the ability, via their software and hardware solutions, to add a second NIC in standby mode. If the primary NIC fails, the second NIC will assume the role of the primary NIC without any client disconnects or performance degradation. All this adds up to more uptime for your servers.