Chapter 14 / 14.2 / 14.2.4

14.2.4 TCP/IP Reference Model

While the OSI model provides a conceptual framework, the TCP/IP Reference Model—also known as the Internet Protocol Suite—defines the architecture used in operational networks worldwide. Developed in the 1970s for the ARPANET, TCP/IP emphasized practical interoperability across heterogeneous networks and was designed to function across diverse transmission technologies.

The TCP/IP model defines four (sometimes five) layers:

1. Network Interface (or Link) Layer: Corresponds broadly to OSI Layers 1–2; defines framing, physical transmission, and local addressing over a specific medium.
2. Internet Layer: Corresponds to OSI Layer 3; defines logical addressing (IP), routing, and packet forwarding across interconnected networks.
3. Transport Layer: Corresponds to OSI Layer 4; provides end-to-end data delivery using protocols such as TCP and UDP.
4. Application Layer: Combines OSI Layers 5–7; includes protocols such as HTTP, SMTP, DNS, FTP, and many others that support user applications and network services.

The OSI and TCP/IP models are best viewed as complementary. OSI provides a systematic conceptual hierarchy for understanding layered communication, while TCP/IP defines the protocols that implement global internetworking in practice. Consequently, we examine the TCP/IP architecture and its protocols in greater detail in Chapter 15.

The reference models described above define how communication functions are logically organized within a networked system. To understand how networks operate in practice, however, we must now examine the specific technologies that implement these layers—beginning with wired local area networks.