Underlay network

Simply put, this is the physical network created by using the networking devices, such as switches, repeaters, routers, and some parameter security devices such as firewalls and so on. This includes the medium on which the packets travel by using electrical, wireless, or optical pulses.

The underlay network will only be called as such when the concept of overlay networks enter the picture. Till such a time, the underlay network is simply referred to as the network. So if you hear the word network, it simply means the underlay network.

In the cloud, not a lot is done on the underlay network (except in some private clouds, where the orchestration of this layer is also included) in the design.