Final answer:
With a single network adapter, an NLB system can function by using multicast mode to map a shared IP address to a multicast MAC address which the adapter listens to along with its own unicast IP.
Step-by-step explanation:
In Network Load Balancing (NLB), when a system has only one physical network adapter, it can still participate in NLB by configuring the adapter to accept traffic for multiple IP addresses through the use of multicast mode. In this mode, the NLB cluster uses a multicast MAC address for all nodes in the cluster, and this multicast MAC address is mapped to a single multicast IP address that is shared among all nodes.
When using a single network adapter, this adapter will handle not only the multicast traffic for the cluster IP address but also the regular unicast traffic that is destined for the server's unique IP address. To differentiate between the traffic for the server itself and the traffic for the NLB cluster, the network adapter listens to both its unicast IP address and the shared multicast IP address.
Most modern switches can support configuring a static entry in their MAC address table to ensure that traffic to the NLB multicast IP address is forwarded to the appropriate ports where the NLB nodes are connected.