Layer 4 load balancing

Layer 4 network load balancing (NLB) works by forwarding traffic to a specific server based upon the port or service the client wants to contact. With NLB, the load balancing appliance sits in between the client workstation and the server. The NLB appliance listens on its own port for requests from the client, and then makes decisions based on certain criteria which server to send them on to. The criteria could be traffic rate (i.e. if a particular server was busier than another), or it could simply be if the original server was no longer online or responding to requests.

This process is completely transparent to the clients requesting the service, they simply make the request and the NLB appliance routes the request to the appropriate server. The chosen server then replies directly to the client and offers the service.