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

Defining Networks with the OSI Model | 45 packets on the network layer, and IP addressing information would be added. The packets would then be sent to the data link layer, where the network adapter would encapsulate them into frames of data. Then, at the physical layer, the network adapter would break the frames up into a serial bit stream to be sent over the cable media. When the serial bit stream arrived at the destination client computer via the web browser, it would be reconfigured by the client’s network adapter as frames of information. The header information of the frames would be checked for authenticity and stripped, leaving the pack- ets to be sent to the operating system. The operating system would then put these packets together to form the web page that is displayed on your computer screen. Of course, this all happens 10,000 times faster than explained here, and it happens many times each second. For instance, if your computer has a 100 Mbps connection, it can take in about 12 MB maxi- mum of data per second. Packets of information are variable in size, and they can be between approximately 60 and 1,500 bytes. Say that you are downloading a large file. This file will be broken down into the largest packet size possible, around 1,500 bytes. We can therefore calculate that an average computer can take in 8,000 packets of data per second. By the way, most client computers probably do not take advantage of this maximum data throughput, but servers and power workstations do. Table 2-3 reviews the OSI layers and shows the corresponding devices, protocols, and network standards that apply to each layer. Table 2-3 The OSI model layers and corresponding components L AYER P ROTOCOL D EVICE 7. Application FTP, HTTP, POP3, SMTP Gateway 6. Presentation Compression, Encryption NA 5. Session LogonLogoff NA 4. Transport TCP, UDP NA 3. Network IP, ICMP, ARP, RIP Routers 2. Data Link 802.3, 802.5 NICs, switches, bridges, WAPs 1. Physical 100BASE-T, 1000BASE-X Hubs, patch panels, RJ45 jacks Although the OSI model is a reference model, the TCPIP model also known as the DoD model or Internet model is more descriptive, defining principles such as “end-to-end” and “robustness,” which describe strong endpoint connections and conservative transmission of data. This model is maintained by the Internet Engineering Task Force IETF. The four layers in the TCPIP model are as follows: • Layer 1: Data link layer also simply known as the link layer • Layer 2: Network layer also known as the Internet layer • Layer 3: Transport layer • Layer 4: Application layer The OSI physical layer is skipped altogether, and the application layer comprises the OSI application, presentation, and session layers. CERTIFICATION READY How can you define the TCPIP model? 3.1 Defining the TCPIP Model The TCPIP or TCP model is similar to the OSI model. It is often used by software manufacturers who are not as concerned with how information is sent over physical media, or how the data link is actually made. This model is composed of only four layers. 46 | Lesson 2 Programmers utilize the TCPIP model more often than the OSI model, whereas network administrators usually benefit to a higher degree from the OSI model. Programmers are generally interested in the interfaces made to the application and transport layers. Anything below the transport layer is taken care of by the TCPIP stack within the operating system, which is set in stone. Programs can be made to utilize the TCP stack, but not to modify it. Again, as a networking person, you will most often refer to the OSI model, but you should know the layers of the TCP model in case you need to interface with programmers and devel- opers, especially programmers and developers of Microsoft products. S K I L L S U M M A R Y I N THIS LESSON , YOU LEARNED : • To understand the OSI model by defining each of the layers from a theory perspective and with hands-on labs. • To be able to separate the functions of the lower levels of the OSI, or the communications subnetwork, from the upper levels where message creation begins. • To understand the differences between layer 2 and layer 3 switches, and to gain a basic understanding of how they operate. • To differentiate between the OSI model and the TCP model. Multiple Choice Circle the letter that corresponds to the best answer. 1. How many layers are incorporated in the OSI model communications subnetwork? a. 2

b. 7 c. 3

d. 4

2. Which of the following layers deals with the serial transfer of data? a. Physical

b. Data link c. Network

d. Session

3. You need to install a router on your company’s network that will allow access to the

Internet. What layer of the OSI does this device reside on? a. Physical

b. Data link c. Network

d. Transport

4. You run a netstat –an command in the command prompt and notice many connections being

made that say TCP in the left-most column. What layer of the OSI is TCP referring to? a. Layer 1

b. Layer 2 c. Layer 3

d. Layer 4

■ Knowledge Assessment Defining Networks with the OSI Model | 47 5. You suspect a problem with your computer’s network adapter and its ability to send the correct frames of data that correspond with the network architecture used by the rest of your computers. What layer should you attempt to use as a troubleshooting starting point? a. Physical

b. Data link c. Network

d. Transport

6. A standard such as 100BASE-T refers to which OSI layer? a. Physical

b. Data link c. Network

d. Transport

7. Almost all of your users connect to Web sites with Internet Explorer. They usually type domain names such as www.microsoft.com. What protocol is initiated by default when they press Enter after typing the domain name? a. FTP

b. HTTPS c. HTTP

d. HTP

8. You need to find out the MAC address of your director’s computer. He has given you

permission to access his computer. You access the command prompt. What command should you type to see the computer’s MAC address?

a. ipconfig b. ipconfigall

c. arp d. netstat -an

9. You need to find out the MAC addresses of all the computers that a particular user’s

computer has connected to in the recent past. What command should you use to accom- plish this?

a. ping 127.0.0.1 b. netstat -a

c. arp -a d. arp -s

10. You have been instructed to capture and analyze packets on a server. What tool will allow

you to do this? Select the two best answers. a. Protocol analyzer

b. Command Prompt c. netstat -an

d. Wireshark

Fill in the Blank Fill in the correct answer in the blank space provided.

1. The manager of IT asks you ping his laptop to see whether your computer can find it on

the network. In this scenario, the ____________ protocol is being implemented. 2. A ____________ switch is one that uses logical addressing to determine data paths. 3. Ports 1024–49,151 are ports used by vendors for proprietary applications. They are known as ____________ ports. 4. Port ____________ is used by the File Transfer Protocol.