An Azure load balancer operates at Layer-4 and provides high availability by distributing incoming traffic among healthy virtual machines. A load balancer health probe monitors a given port on each virtual machine and only distributes traffic to an operational virtual machine.

A typical load balancer configuration can be illustrated like the following diagram:

Although the preceding diagram looks pretty simple, there is actually quite a bit going on behind the scenes; a fair of configuration is necessary to make things work.

The deployment of a load balancer requires you to define a frontend IP configuration that contains one or more public IP addresses, which then allows the load balancer (as well as any applications and VMs being load balanced) to be accessed over the internet.

Since virtual machines are connected to a load balancer using their virtual network interface cards, a backend address pool must be created that contains the IP addresses of the virtual NICs connected to the load balancer.

Traffic flow to the load-balanced virtual machines is controlled with defined load balancer rules for specific ports and protocols.

As we've already mentioned, there is quite a bit going on under the hood when it comes to load balancers in Azure.