Definition of protocol in The Network Encyclopedia.

What is Protocol?

A set of rules for sending information over a network. Protocols can include rules concerning any or all of the following functions:

  • Data transmission mechanisms
  • Communication session initialization and termination
  • Addressing and routing
  • Authentication and verification
  • Encryption and compression
  • Error correction

Protocols are usually classified according to the layer they correspond to in the Open Systems Interconnection (OSI) reference model for networking. Types of protocols include the following:

  • Data-link protocols:
    Govern the framing of data, physical addressing of network nodes, and media access control methods. For local area networks (LANs), these primarily include Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI). For wide area networks (WANs), they include Point-to-Point Protocol (PPP), High-level Data Link Control (HDLC), frame relay, Asynchronous Transfer Mode (ATM), and X.25.


  • Network protocols:
    Handle link services and are responsible for addressing, routing, and error checking. Examples include NetBEUI, Internetwork Packet Exchange (IPX), NWLink, and Internet Protocol (IP).


  • Transport protocols:
    Enable the establishment of sessions and ensure reliable flow of data. Examples include NetBEUI, Sequenced Packet Exchange (SPX), NWLink, and Transmission Control Protocol (TCP).


  • Application layer protocols:
    Enable applications to access network services. Examples include Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Telnet, Simple Mail Transfer Protocol (SMTP), Simple Network Management Protocol (SNMP), Network News Transfer Protocol (NNTP), X.400, X.500, Server Message Block (SMB), and Network File System (NFS).


Some protocols have been developed by specific vendors and then accepted as de facto standards by the industry, while others were initially formulated by independent standards bodies and then accepted and implemented by vendors. The most widely implemented protocols are those relating to TCP/IP and the Internet.