Link, Network, and Transport Layers - Quiz Explanation

The correct answers are indicated below, along with the text that explains the correct answers.
 
1. Which of the following handles the link, network, and transport layers on a UNIX system?
Please select the best answer.
  A. The client and server processes
  B. The portmapper process
  C. The kernel
  D. An HTTP client process
  The correct answer is C.
The kernel handles the link, network, and transport layers on a UNIX system. The kernel is responsible for the proper handling of processes that address networking requirements, including all network activity at the link, network, and transport layers. Although protocols such as UDP, TCP, and IP prepare packets, the kernel handles protocols on the computer.

2. Give two examples of link-layer protocols.
Please select the best answer.
  A. FDDI and PPP
  B. UDP and TCP
  C. UDP and PPP
  D. HTTP and FTP
  The correct answer is A.
Two examples of link-layer protocols are FDDI and PPP. FDDI is used for fiber optics networks and PPP is used across phone lines and other point-to-point connections. The key to understanding link-layer protocols is that they focus specifically on getting packets out of the host's NIC and on to the network wire.

3. Bill sends Jane an email. His address is 10.100.100.34. Jane's is 192.168.5.35. These are completely different subnets. Even though they are on different subnets, their router enables them to communicate, and the email is delivered. Which of the answers below best describes the routing process?
Please select the best answer.
  A. The process of establishing communication between machines at the physical-connection level, via routing tables
  B. The process of directing packets among a choice of network paths via routing tables, based on the intended destination
  C. The process of using RIP and routing tables in directing processes over the Internet
  D. The process of establishing tables for network clients and servers to use for communication using routers
  The correct answer is B.
Routers use routing tables, which enable packets to be sent to the correct destination. Among other things, the network layer is responsible for routing, which in regard to TCP/IP is based solely on IP addresses. A is incorrect because IP does not care about physical connectivity, which is taken care of at the link level. C is incorrect because RIP is an (older) routing protocol, but is designed for routers to exchange routing tables with each other, not for directing processes or packets. Finally, answer D is incorrect because it describes what routers do when they exchange routing tables, rather than describing the fundamental concept of routing.

4. James has just sent Subash a file over the Internet using FTP, an application-layer protocol. However, at what layer of the TCP/IP stack would James' FTP process reach the destination process for Subash's FTP server?
Please select the best answer.
  A. The data link layer
  B. The link layer
  C. The transport layer
  D. The network layer
  The correct answer is C.
The transport layer ensures that traffic sent by a process on the sending machine reaches a similar process on the intended receiving machine. The data link layer is proposed by the OSI/RM, not by the four-layer TCP/IP model. The link layer has to do with packets flowing onto the wire, and the network layer is largely concerned with routing.

5. What is a netmask (aka, a subnet mask)?
Please select the best answer.
  A. A separate IP address with all host bits set to zero, which determines its ability to participate on a network
  B. The host portion of an IP address that determines its identity on a network
  C. The network portion of an IP address that tells how it can participate on a network
  D. A separate 32-bit number that tells the host which portion of its IP address indicates the host and which portions indicate the network
  The correct answer is D.
A netmask is a separate 32-bit number that tells the host which portion of its IP address indicates the host and which portions indicate the network. The netmask determines the difference between the host and network portions of an IPv4 address. Any netmask has a one bit set in each position of the network address, and a zero bit set in each position of the host address. A netmask is usually written in dotted decimal form. For example, for a standard class A IP address, the netmask would be 255.0.0.0. For a standard class C IP address, the netmask would be 255.255.255.0. A network host uses the netmask to manage its participation on an IP network.

6. In an IP network, what is the chief difference between the link layer and the network layer?
Please select the best answer.
  A. The link layer provides a language that allows clients and servers to communicate with each other, whereas the network layer directs information between IP addresses.
  B. The link layer establishes communication between machines on the same physical segment, whereas the network layer provides addressing between different networks.
  C. The link layer moves data between machines across different segments, whereas the network layer provides addressing between machines on the same network.
  D. The link layer directs information between processes, whereas the network layer provides a language that allows clients and servers to communicate with each other.
  The correct answer is B.
The chief difference between the link layer and network layer is that the link layer establishes communication between machines on the same physical segment, whereas the network layer provides addressing between different networks. The link-layer protocols are typically concerned only with local segments. The network layer adds routing capability, which includes IP addresses.

7. A standard IPv4 address is divided into four parts.
For example, consider the following IP address: 192.168.4.34. To what do the four parts of an IP address correspond?
Please select the best answer.
  A. The four layers of the TCP/IP protocol stack
  B. The type of IP address: Class A, Class B, Class C, and reserved
  C. Each eight-bit section of the 32-bit address
  D. Four different Internet protocols
  The correct answer is C.
An IP address is broken into four bits, each bit represented by one numeric value, separated by a dot. Each value within a “dotted quad” section will be an integer between 0 and 255. In the above question, 192 represents the value for the first bit, whereas 34 represents the value of the fourth. Together, they comprise a 32-bit IP address.

8. How does the transport-layer protocol add reliability to the process-level addressing capabilities?
Please select the best answer.
  A. By moving data from a serial protocol, through a modem, and across the telephone
  B. By creating a reliable connection on top of the underlying network- and link-layer protocols
  C. By adding routing capability to move data from one network to another
  D. By using the loopback address to connect to the machine that originally sent a packet
  The correct answer is B.
The transport-layer protocol adds reliability to the process-level addressing capabilities by creating a reliable connection on top of the underlying network- and link-layer protocols. TCP is a transport-layer protocol. The responsibility of the transport layer is to ensure that traffic intended for a specific process actually reaches that process in a reliable manner.