Windows Network Load Balancing (NLB) is a feature that distributes network traffic among multiple servers or virtual machines within a cluster to avoid overloading any one host and improve performance.
The Network Load Balancing feature uses the TCP/IP networking protocol to route traffic to different hosts based on a user's settings and a proprietary Microsoft distribution algorithm. The feature is available in Microsoft Windows Server 2008, 2008 R2, 2012 and 2012 R2 operating systems.
Network Load Balancing has several potential use cases and advantages. By distributing network traffic across multiple servers or virtual machines, traffic can be processed faster than in a scenario in which all traffic flowed through a single server. The feature can also enable an organization to quickly scale up a server application (such as a Web server) by adding hosts and then distributing the application's traffic among the new hosts. Similarly, if demand decreases, servers can be taken offline and the feature will balance traffic among the remaining hosts. Network Load Balancing can also ensure network traffic is re-routed to remaining hosts if one or more hosts within the cluster fail unexpectedly. A Network Load Balancing cluster can scale up to 32 servers.
Systems administrators and networking administrators can manage the Network Load Balancing feature through Microsoft's NLB Manager or with PowerShell cmdlets.