Introduce the application layer. the application layer is an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet Protocol (IP) computer network. Application layer protocols use the underlying transport layer protocols to establish host-to-host connections.-Introduce the application layer.　the application layer is an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet Protocol (IP) computer network. Application layer protocols use the underlying transport layer protocols to establish host-to-host connections.