Application layer definition
An application layer is the topmost OSI (Open System Interconnection) network layer. Because this layer offers a variety of options for working with data, it's possible for virtually any user to have easy access to the network. This layer also sends requests to the presentation layer below it to collect various kinds of data. Common web application services are exposed through the application layer interface, which facilitates direct communication across applications. In an open system, this is the top level, and it offers services that are used by the applications themselves.
Application layer protocols
- TELNET. A person can easily manage files while transferring them over the web. This opens up the Telnet server's resources to users connecting through Telnet. Telnet connects to the Internet over TCP/UDP port 23.
- DNS. Converts user-selected domain names into IP addresses. DNS traffic goes through TCP/IP protocol on port 53.
- DHCP. Provides IP addresses to hosts. Ports 67 and 68 are used by the DHCP service.
- FTP. Transfers files between devices. FTP makes remote file sharing easy and dependable. FTP data access and control use ports 20 and 21.
- SMTP. Transfers email between users. End users easily send emails using SMTP. Uses port numbers 25 and 587.
- HTTP. Created for web browser-server connections, although it can be used for other purposes. HTTP is a stateless protocol, meaning the server does not maintain prior client requests. Uses port number 80.
- NFS. Allows remote hosts to mount and interact with networked file systems as if they were locally mounted. Uses port number 2049.
- SNMP. Polls network devices to the management station at regular or random intervals to collect data. Uses port numbers 161 (TCP) and 162 (UDP).
