Layer 4 Load Balancing NAT Mode

NAT stands for Network Address Translation.

In the NAT mode, the load-balancer will route traffic between user and server by changing destination IP address of the packets.

TCP Connection Overview

TCP connection is established between the client and the server. The load-balancer just ensures a client is always forwarded to the same server.

Layer4 NAT TCP Connection

Data Flow

As shown below, the clients get connected to the service VIP. The load-balancer chooses a server in the pool then forwards packets to it by changing destination IP address.

Layer4 NAT Data Flow

Pros and Cons


  • Fast load balancing
  • Easy to deploy


  • Infrastructure intrusive: need to change the default gateway of the servers
  • The server default gateway must use the load balancer, in order to do reverse NAT operation
  • Output bandwidth is limitated by load balancer output capacity

When Use This Architecture

  • When output capacity of the load balancer won’t be a bottleneck in a near future
  • When nothing but the default gateway of the servers can be changed