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The origin of many networking myths can be traced back to the Open Systems Interconnection (OSI) reference model developed by Charles Bachman of Honeywell and formalized by the International Organization for Standardization (ISO). The ISO intended the OSI model to be a standard framework for data networks. It describes a set of “activities necessary for systems to interwork using communication media” (ISO/IEC 7498-4). The model organizes these activities or functions into the following seven layers:

 7. Application

 6. Presentation

 5. Session

 4. Transport

 3. Network

 2. Data Link

 1. Physical

The seven layers are taught zealously in most introductory networking courses. You may have had them permanently drilled into your head with the help of one or two fun little mnemonics! (My favorite is “All people seem to need data processing.”) As we discuss the functions of the different layers, keep in mind that the layers of the OSI model are arbitrary. They're not written on stone tablets, nor are they the result of a rigorous scientific process that conclusively proved that the perfect network has these seven layers. The ISO arrived at each layer by attempting to group similar network functions together in a layer and then organizing the layers in a hierarchical fashion so that each layer of functions is dependent on the one below it. This led to impressive results in layers 1–4 (the lower layers) and utter confusion in layers 5–7 (the upper layers).

Table 1.1 shows what common protocols fall into each of the lower layers.

Table 1.1 The lower layers and their associated protocols

Layer	Name	Example protocols
1	Physical	Thicknet (10BASE5)
		Thinnet (10BASE2)
		1000BASE-T
		T1/E1
2	Data Link	IEEE 802.3/Ethernet II (DIX)
		Point-to-Point Protocol (PPP)
		High-Level Data Link Control (HDLC)
3	Network	IPv4
		IPv6
4	Transport	TCP
		UDP