Figure 71 OSI Reference Model

OSI Reference Model

Internet Protocol Suite

Application

Presents, tiori

Session

Transport

Network

Link

Physical

Internet Protocol Suite

FTP, Telnet, SMTP, SNMP

NFS

XDR

RPC

TCP, UDP

Routing Protocols

IF

ICMP

ARP. RARP

Not Specified

In addition, each layer of the OSI reference model provides services to the layer above it (Layer 5 to Layer 6, Layer 6 to Layer 7, and so on) and requests certain services from the layer directly below it (5 to 4, 4 to 3, and so on).

This layered approach enables each layer to handle a small piece of information, make any necessary changes to the data, and add the necessary functions for that layer before passing the data along. Data becomes less human-like and more computer-like the further down the OSI reference model it traverses, until it becomes 1s and 0s (electrical impulses) at the physical layer. Figure 7-1 shows the OSI reference model.

The primary focus of this chapter is to discuss the application, presentation, session, transport, network, data link, and physical layers. Understanding these layers allows you to understand how IP routing works and how IP is transported across various media occurring at Layer 2 and Layer 1.

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