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Internet Layer

The Internet Layer of the TCP/IP model aligns with the Layer 3 (Network) of the OSI model. This is where IP addresses and routing live. When data is transmitted from a node on one LAN to a node on a different LAN, the Internet Layer is used. IPv4, IPv6, ICMP, and routing protocols (among others) are Internet Layer TCP/IP protocols.

Exam Warning
Layer 3 of the OSI model is called “Network.” Do not confuse OSI’s Layer 3 with the “Network Access” TCP/IP layer, which aligns with Layers 1 and 2 of the OSI model.

Host-to-Host Transport Layer

The Host-to-Host Transport Layer (sometimes called either “Host-to-Host” or, more commonly, “Transport” alone; this book will use “Transport”) connects the Internet Layer to the Application Layer. It is where applications are addressed on a network, via ports. TCP and UDP are the two Transport Layer protocols of TCP/IP.

Application Layer

The TCP/IP Application Layer combines Layers 5 through 7 (Session, Presentation, and Application) of the OSI model. Most of these protocols use a client-server architecture, where a client (such as ssh) connects to a listening server (called a daemon on UNIX systems) such as sshd. The clients and servers use either TCP or UDP (and sometimes both) as a Transport Layer protocol. TCP/IP Application Layer protocols include SSH, Telnet, and FTP, among many others.

Encapsulation

Encapsulation takes information from a higher layer and adds a header to it, treating the higher layer information as data. It is often said, “One layer’s header is another layer’s data”[^2]. For example, as the data moves down the stack, application layer data is encapsulated in a Layer 4 TCP segment. That TCP segment is encapsulated in a Layer 3 IP packet. That IP packet is encapsulated in a Layer 2 Ethernet frame. The frame is then converted into bits at Layer 1 and sent across the local network. Data, segments, packets, frames, and bits are examples of Protocol Data Units (PDUs).

Note
The mnemonic “SPF10” is helpful for remembering PDUs: Segments, Packets, Frames, 1s and 0s.

The reverse of encapsulation is called de-multiplexing (sometimes called de-encapsulation). As the PDUs move up the stack, bits are converted to Ethernet frames, frames are converted to IP packets, packets are converted to TCP segments, and segments are converted to application data.