Protocols are internationally agreed-upon communication frameworks and standards that govern communications across computers or computer systems. Some of the most common protocols are Transmission Control Protocol/Internet Protocol (TCP/IP), User Datagram Protocol (UDP), Internet Control Message Protocol (ICMP), and Hypertext Transfer Protocol (HTTP).

Protocols may carry certain message types or used for specific purposes. Protocol communications may occur within certain layers of communications. Such layers of communications and the types of communications that occur within them have been represented in the industry in the following two commonly used representations: the Open Systems Interconnection (OSI) model and the TCP/IP model.

OSI layers 7 application, 6 presentation, and 5 session are equivalent to TCP/IP layer 4 applications. OSI layer 4 transportation is equivalent to TCP (host-to-host) layer 3. OSI layer 3 network is equivalent to IP (Internet) layer 2. OSI layer 2 data link and layer 1 physical are equivalent to TCP/IP layer 1 network access (usually Ethernet).

Different protocols are used at different levels of the OSI and TCP/IP models. For example, the ICMP protocol is used at the network layer of the OSI Model and the internet layer of the TCP/IP model. TCP is used at the transport layer for both models. IP, or the Internet Protocol, is one of the foundational protocols for communicating over the internet.

TCP is used to break down information into packets and then transmit them across the network. UDP is used when less information is transferred or when a connectionless method is preferred, while ICMP is used primarily for small management or diagnostic messages.

Protocols usually communicate over specific TCP ports. For example, SMTP (Simple Mail Transfer Protocol) operates over port 25 and is used to transfer mail. TCP ports 80 and 443 are used for HTTP and HTTPS (Hypertext Transfer Protocol and Secure Hypertext Transfer Protocol) to manage web transmission.


Ports range from 0 to 65535; ports under 1024 are considered unregistered ports. It is best to use higher ports for troubleshooting and other configurations to avoid conflicting with ports that are designated for specific protocol communication.

There are several hundreds of protocols that operate over many ports. Organizations should always be aware of the protocols in use to ensure a secure enterprise environment.