TCP/IP Model Layers

The TCP/IP model uses four layers that logically span the equivalent of the top six layers of the OSI reference model; this is shown in Figure. (The physical layer is not covered by the TCP/IP model because the data link layer is considered the point at which the interface occurs between the TCP/IP stack and the underlying networking hardware.) The following are the TCP/IP model layers, starting from the bottom.

Network Interface Layer
As its name suggests, this layer represents the place where the actual TCP/IP protocols running at higher layers interface to the local network. This layer is somewhat “controversial” in that some people don't even consider it a “legitimate” part of TCP/IP. This is usually because none of the core IP protocols run at this layer. Despite this, the network interface layer is part of the architecture. It is equivalent to the data link layer (layer two) in the OSI Reference Model and is also sometimes called the link layer. You may also see the name network access layer.

On many TCP/IP networks, there is no TCP/IP protocol running at all on this layer, because it is simply not needed. For example, if you run TCP/IP over an Ethernet, then Ethernet handles layer two (and layer one) functions. However, the TCP/IP standards do define protocols for TCP/IP networks that do not have their own layer two implementation. These protocols, the Serial Line Internet Protocol (SLIP) and the Point-to-Point Protocol (PPP), serve to fill the gap between the network layer and the physical layer. They are commonly used to facilitate TCP/IP over direct serial line connections (such as dial-up telephone networking) and other technologies that operate directly at the physical layer.

TCP/IP Protocol Stack
OSI Ref. Layer No.
OSI Layer Equivalent
TCP/IP Layer
TCP/IP Protocol Examples
5,6,7
Application, session, presentation
Application
NFS, NIS, DNS, LDAP, telnet, ftp, rlogin, rsh, rcp, RIP, RDISC, SNMP, and others
4
Transport
Transport
TCP, UDP, SCTP
3
Network
Internet
IPv4, IPv6, ARP, ICMP
2
Data link
Data link
PPP, IEEE 802.2
1
Physical
Physical network
Ethernet (IEEE 802.3), Token Ring, RS-232, FDDI, and others

7 Layers of OSI Model


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