Single network adapter

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Single network adapter

Network Load Balancing can operate in two modes: unicast and multicast.

In unicast mode, the cluster's media access control (MAC) address is assigned to the computer's network adapter, and the network adapter's built-in MAC address is not used.

In multicast mode, the cluster's MAC address is assigned to the computer's network adapter, but the network adapter's built-in address is retained so that both addresses are used, the first for client-to-cluster traffic and the second for network traffic specific to the computer.

The following describes the principal characteristics of each mode when a single network adapter is used.

Single network adapter in unicast mode

  • The adapter's own, unique MAC address is disabled.

    The cluster MAC address (which Network Load Balancing automatically generates) replaces this address.

  • The adapter becomes, in effect, the cluster adapter.

  • Both the dedicated IP address and the cluster IP address resolve to the cluster MAC address.

  • Because all cluster hosts share the same MAC address, and because the original MAC address is not used, ordinary network communication between this host and the other cluster hosts is not possible. However, the computer can still handle traffic originating from outside the subnet in which the cluster is located, and from inside the subnet if the IP datagram does not carry the same MAC address as on the cluster adapter. For more information, see Address Resolution Protocol (ARP).

Single network adapter in multicast mode

  • Network Load Balancing automatically generates a cluster MAC address for the network adapter.

  • The adapter's own, unique MAC address is retained.

  • The cluster IP address resolves to the cluster MAC address.

  • The dedicated IP address resolves to the original MAC address.


  • Network Load Balancing does not support a mixed unicast/multicast environment within a single cluster. Within each cluster, all network adapters in that cluster must be either multicast or unicast; otherwise, the cluster will not function properly.

    However, there is no restriction on the number of network adapters: Different hosts can have a different number of adapters.

  • To start Network Load Balancing Manager, open a Command prompt. From the command window, type NLBMgr, and then press ENTER.

Unicast support is enabled by default. To enable multicast support, see Enable multicast support.

If multicast support is enabled, a single network adapter can handle both client-to-cluster traffic and dedicated traffic without difficulty. However, the use of a second network adapter might improve performance by handling dedicated traffic concurrently with the original network adapter.

Multicast support, with or without multiple network adapters, has several advantages, such as eliminating the inherent limitations of a single network adapter, as described in Single network adapter limitations. Network Load Balancing handles the resolution of the cluster's primary IP address to its associated multicast cluster MAC address within the ARP on your router. In rare cases, the use of a multicast MAC address might not be supported by the ARP implementation on a given router. If this problem arises, the cluster will not be accessible from outside the local subnet. To correct this problem, you must create a static ARP entry within the router. Refer to the documentation for your router to determine how to create a static ARP entry. For more information, see Network Load Balancing system requirements.

If you disable multicast support (causing the host to revert to unicast mode), Network Load Balancing automatically instructs the driver belonging to the cluster adapter to override the adapter's unique, built-in network address and to change its MAC address to the cluster's MAC address. This is the address used on all cluster hosts. You do not need to manually configure the network adapter to recognize this address. (Note that some network adapters do not support changing their MAC addresses. If you experience this problem, you must install a network adapter that does.)

If you are using multiple network adapters, see Multiple network adapters.