The application layer is the topmost layer of the protocol hierarchy. It is the layer where actual communication is initiated. It uses the services of the transport layer, the network layer, the data link layer, and the physical layer to transfer data to a remote host. Two remote application processes can be in one of the two different modes:
- a. Peer-to-peer mode, which means that the two processes are executing at the same level and they exchange data using some shared resource.
- b. Client–server mode, which means that one process acts as a client that is requesting some resource from the other process that is acting as the server that is providing the service.
Many protocols have been defined for the application and they include the following:
- a. Dynamic host configuration protocol (DHCP) is a protocol that automatically provides an IP address to a host. It also provides all related configuration information, such as subnet mask and default gateway, to the host. It is a client–server protocol that has the port number 546 for the client and 547 for the server.
- b. Domain name system (DNS) is used to resolve human-readable host names into IP addresses. In particular, an e-mail address such as email@example.com cannot be routed as is; it needs to be mapped to an IP address, and this is the role of the DNS. The port number for DNS is 53.
- c. File transfer protocol (FTP) is a standard network protocol used to transfer computer files ...