TCP/IP – Transmission Control Protocol / Internet Protocol

The internet uses the open standard TCP/IP model created by the Department of Defense. It was designed on the basis that it could survive any conditions including nuclear war.

The TCP/IP protocol stack has four layers, some of which have the same names as OSI layers, however do not confuse the two because they have different functions.

Because the OSI reference model is an easier way to educate and troubleshoot networks it is used in Cisco’s CCNA curriculum for teaching purposes instead of the TCP/IP model. However the TCP/IP protocols are related to the OSI model, because these are the protocols that are in use today.

Brief explanations of the TCP/IP layers follow.

OSI vs TCP/IP Models

TCP/IP protocols are the standards upon which the Internet was built. No networks are built around the OSI model even though the OSI model is still used as a reference tool to guide design and thinking.

TCP/IP Application Layer

The TCP/IP application layer handles all high level protocols that the Application, Presentation and Session layers of the OSI model handle, including encoding and dialogue control issues.

TCP/IP Transport Layer

The TCP/IP transport layer uses two protocols, Transport Control Protocol (TCP) and User Datagram Protocol (UDP), which deal with connection-orientated and connection-less communication respectively.

TCP provides reliable, low-error network communication using packet switching, while UDP provides a low overhead connection-less alternative.

TCP/IP Internet Layer

The Internet layer of TCP/IP is based entirely around one protocol, the Internet Protocol or IP. This layer has very similar functions to the OSI Network Layer. It’s primary function is to negotiate the best path to a destination.

TCP/IP Network Layer

The TCP Network Layer combines the tasks of the OSI Physical and Data Link layers into one layer.

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