Above the transport layer lies the application layer, which defines application message formats and communication semantics. The web uses a client server application protocol called Hypertext Transfer Protocol (HTTP) .
There exist various kinds of CNs, differing in the extend to which they handle these tasks and in the mechanisms they use to do so. There are many possible ways to classify them. In this article, we use a classification based on who owns and administrates the CN. We thus find three categories: CNs owned by network operators, content providers, and users.
Some switches are called ``L4 switches'' (4 is the number of the transport layer in the OSI reference model) meaning they look at network and transport layer information in the first packet of a connection to decide to which physical server the incoming connection should be handed. They establish a state associating the connection with the chosen physical server and use it to relay all packets of the connection. The exact way the packets are sent to the physical servers vary. It usually involves a form of mangling of IP and TCP headers in the packets (like NAT -- Network Address Translation -- does) or IP encapsulation. These tricks are not not necessary if all the physical servers live on the same LAN (local area network).