Summary

40 IT Essentials: PC Harware and Software Companion GuideTable 8-6TCP/IP ModelTCP/IP Model Layer DescriptionApplication 4 Where high-level protocols such as SMTP and FTP operateTransport 3 Where flow-control and connection protocols existInternet 2 Where IP addressing and routing take placeNetwork access 1 Where MAC addressing and physical network componentsexistA message begins at the top layer, the application layer, and moves down the TCP/IP layersto the bottom layer, the network access layer. Header information is added to the messageas it moves down through each layer and then is transmitted. After reaching the destination,the message travels back up through each layer of the TCP/IP model. The header informationthat was added to the message is stripped away as the message moves up through thelayers toward its destination.Application ProtocolsApplication layer protocols provide network services to user applications such as webbrowsers and e-mail programs. Here are some of the application protocols that operate atthe TCP/IP application layer:■■■■■■Hypertext Transfer Protocol (HTTP) governs how files such as text, graphics, sound,and video are exchanged on the Internet or World Wide Web (WWW). HTTP is anapplication layer protocol. A web server runs an HTTP service or daemon. A daemon isa program that services HTTP requests. These requests are transmitted by HTTP clientsoftware, which is another name for a web browser.Telnet is an application that you can use to access, control, and troubleshoot remotecomputers and network devices.File Transfer Protocol (FTP) is a set of rules governing how files are transferred. FTPallows multiple simultaneous connections to remote file systems.Simple Mail Transport Protocol (SMTP) provides messaging services over TCP/IPand supports most Internet e-mail programs.Domain Name System (DNS) translates domain names, such as www.cisco.com, to IPaddresses.Hypertext Markup Language (HTML) is a page description language. Web designersuse HTML to indicate to web browser software how the page should look. HTMLincludes tags to indicate boldface and italic type, line breaks, paragraph breaks, hyperlinks,and insertion of tables, among other instructions.