Application layer—Roughly equivalent to the Application, Presentation, and Sessionlayers of the OSI model. Applications gain access to the network through this layer,via protocols such as HTTP (Hypertext Transfer Protocol), FTP (File TransferProtocol), Telnet, NTP (Network Time Protocol), DHCP (Dynamic HostConfiguration Protocol), and PING (Packet Internet Groper), to name only some.● Transport layer—Roughly corresponds to the Transport layer of the OSI model. Thislayer holds the Transmission Control Protocol (TCP) and User Datagram Protocol(UDP), which provide flow control, error checking, and sequencing.● Internet layer—Equivalent to the Network layer of the OSI model. This layer holdsthe Internet Protocol (IP), Internet Control Message Protocol (ICMP), Internet GroupManagement Protocol (IGMP), and Address Resolution Protocol (ARP). Theseprotocols handle routing and address resolution.● Network Interface layer (orLink layer)—Roughly equivalent to the Data Link andPhysical layers of the OSI model. Functions in this layer handle formatting of dataand transmission to the network interface.Unlike the OSI model, the TCP/IP model grew to describe how protocols workafterthe protocols were widely in use. For this reason, it is sometimes considered more practical than theOSI model, which is considered more theoretical. Understanding what functions belong toeach layer of the models will come in handy when you’re troubleshooting problems. Forexample, a simple test might indicate that a transmission is breaking down at the Transportlayer. Although you might not know the cause of the problem, narrowing down its scope inthis way will lead you and your colleagues to the next step in assessing and fixing theproblem.
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