Cisco's ICND1 certification exam tests you heavily on the OSI model and the different protocols and devices that run at each layer of the OSI model. The following reviews the OSI model by giving you a description of each layer and examples of protocols and devices that run at each layer.

Layer Description Examples
7.Application Responsible for initiating or services the request. SMTP, DNS, HTTP, and Telnet
6.Presentation Formats the information so that it is understood by the receiving system. Compression and encryption depending on the implementation
5.Session Responsible for establishing, managing, and terminating the session. NetBIOS
4.Transport Breaks information into segments and is responsible for connection and connectionless communication. TCP and UDP
3.Network Responsible for logical addressing and routing IP, ICMP, ARP, RIP, IGRP, and routers
2.Data Link Responsible for physical addressing, error correction, and preparing the information for the media MAC address, CSMA/CD, switches, and bridges
1.Physical Deals with the electrical signal. Cables, connectors, hubs, and repeaters