Implementing Network Load Balancing (NLB) you must choose between unicast and multicast mode, there are a few differences, but the main difference is in the way MAC addresses are implemented.
Unicast - Each NLB cluster node replaces its real (hard coded) MAC address with a new one (generated by the NLB software) and each node in the NLB cluster uses the same (virtual) MAC. Because of this virtual MAC being used by multiple computers, a switch is not able to learn the port for the virtual NLB cluster MAC and is forced to send the packets destined for the NLB MAC to all ports of a switch to make sure packets get to the right destination
Multicast - NLB adds a layer 2 MAC address to the NIC of each node. Each NLB cluster node basically has two MAC addresses, its real one and its NLB generated address. With multicast, you can create static entries in the switch so that it sends the packets only to members of the NLB cluster. Mapping the address to the ports being used by the NLB cluster stops all ports from being flooded. Only the mapped ports will receive the packets for the NLB cluster instead of all ports in the switch. If you don't create the static entries, it will cause switch flooding just like in unicast.
In general, you should enable and use multicast NLB whenever possible. Use unicast mode only if your network equipment—switches and routers—don't support multicast or if they experience significant performance issues when multicast is enabled.