Look for the first HTTP packet in the Protocol column. This should be called GET

44 | Lesson 2 7. Click the ⴙ sign next to Transmission Control Protocol. Here, you will see the out- bound port used by your computer to connect to the web server known as a source port, as well as the inbound port 80 that the web server uses known as a Dst or destination port.

8. Spend some time analyzing the information listed, and match it to the appropriate

layer of the OSI model. Devices known as gateways reside on the application layer. These are not to be confused with gateway devices like routers on the network layer. An application layer gateway is a computer that translates from one protocol suite to another, such as from TCPIP to IPXSPX. An example, albeit an out of date one, would be Client Services for NetWare when loaded on a Windows client computer. Reviewing the OSI Layers CERTIFICATION READY What do you need to know to review all of the OSI layers? 3.1 Application Presentation Transmitting Computer Session Transport Network Data Link Physical Application Presentation Receiving Computer Session Transport Network Data Link Physical Figure 2-8 OSI layers revisited In general, data transactions start at the sending computer, travel down the OSI layers start- ing with the application layer and ending with the physical layer, are transmitted across the physical medium be it wired or wireless, and travel back up the layers of the OSI model at the receiving computer. For example, if you wanted to connect to a Web site, you would type the name of the site in your web browser’s address field. Then, when you press Enter, the HTTP protocol would take effect at the application layer. The packets of data would be compressed with gzip and possibly encrypted HTTPS by way of SSL or TLS at the presentation layer. The web server would acknowledge the session with the client web browser at the session layer. The information would then be transmitted as TCP information on the transport layer, where ports are also selected. The TCP information would be broken up into easy-to-send The OSI model contains seven layers, each of which work collectively to define the trans- mission of data from one computer to another. The mnemonic device All People Seem To Need Data Processing can help you memorize the layer order. Although earlier in the lesson, we defined each of the OSI layers starting at the bottom, the physical layer, and moving upward from there, quite often, you will see the layers listed from the top down, with the application layer at the top and the physical layer at the bottom, as shown in Figure 2-8. However, in Wireshark and other protocol analyzers, the physical layer will be displayed at the top. It all depends on what application or technical document you are looking at, so be ready to encounter both orientations.