Layer 4 Load Balancer
A typical network router simply sends incoming packets onto the appropriate IP address on its network. A layer 4 router, more correctly a NAT with port and transaction awareness, uses a little trickery and sends incoming packets to one or more machines that are hidden behind a single IP address.
The layer 4 refers to the 4 layer OSI model. The router is on the Transport Layer and makes decisions on where to send the packets. Modern load balancing routers can use different rules to make decisions on where to route traffic. This can be based on least load, or fastest response times, or simply balancing requests out. This is also a redundancy method, so if one machine is not up, the router will not send traffic to it.
Read more about this topic: Multilayer Switch
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