The TCP/IP Guide - TCP/IP Protocols

TCP/IP includes two protocols at the network interface layer, SLIP and PPP, which are described in Table 19.

**Table 19: TCP/IP Protocols: Network Interface Layer (OSI Layer 2)**
Protocol Name	Protocol Abbr.	Description
Serial Line Internet Protocol (SLIP)	SLIP	Provides basic TCP/IP functionality by creating a layer-two connection between two devices over a serial line.
Point-to-Point Protocol	PPP	Provides layer-two connectivity like SLIP, but is much more sophisticated and capable. PPP is itself a suite of protocols (“sub-protocols” if you will) that allow for functions such as authentication, data encapsulation, encryption and aggregation, facilitating TCP/IP operation over WAN links.

Table 20 describes ARP and RARP, the “oddballs” of the TCP/IP suite. In some ways they belong in both layer two and layer three, and in other ways neither. They really serve to link together the network interface layer and the internet layer. For this reason, I really believe they belong between these two and call them “layer connection” protocols. See the section devoted to these protocols and their unique layer for more on this issue.

**Table 20: TCP/IP Protocols: Network Interface / Network Layer (“OSI Layer 2/3”)**
Protocol Name	Protocol Abbr.	Description
Address Resolution Protocol	ARP	Used to map layer three IP addresses to layer two physical network addresses.
Reverse Address Resolution Protocol	RARP	Determines the layer three address of a machine from its layer two address. Now mostly superseded by BOOTP and DHCP.

TCP/IP Architecture and the TCP/IP Model

TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)

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