This chapter covers the following subjects:
Typical Features of OSI Layer 3
IP Addressing Fundamentals
Network Layer Utilities
IP Routing and Routing Protocols
The OSI model assigns the functions of path selection and logical addressing to the OSI network layer (Layer 3). Path selection includes the process of learning all the paths, or routes, in a network and then forwarding packets based on those paths or routes. Often the terms path selection and routing are used interchangeably. In most Cisco documentation and in this book, routing is the more popular term.
In this chapter, you will learn about the core concepts behind OSI Layer 3. Because CCNA focuses on TCP/IP, you also will learn about the main Layer 3 protocol used by TCP/IP—namely, the Internet Protocol (IP). This coverage includes IP addressing, IP routing, and some protocols useful to IP's effort to deliver packets end to end through a network.
"Do I Know This Already?" Quiz
The purpose of the "Do I Know This Already?" quiz is to help you decide whether you really need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.
The 12-question quiz, derived from the major sections in the "Foundation Topics" portion of the chapter, helps you determine how to spend your limited study time.
Table 5-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.
Table 5-1 "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
|
Foundations Topics Section |
Questions Covered in This Section |
|
Typical Features of OSI Layer 3 |
1, 2, 4, 12 |
|
IP Addressing Fundamentals |
5–9 |
|
Network Layer Utilities |
10, 11 |
|
IP Routing and Routing Protocols |
3 |
NOTE
The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer that you correctly guess skews your self-assessment results and might provide you with a false sense of security.
Which of the following describes the functions of OSI Layer 3 protocols?
Logical addressing
Physical addressing
Path selection
Arbitration
Error recovery
Imagine that PC1 needs to send some data to PC2, and PC1 and PC2 are separated by several routers. What are the largest entities that make it from PC1 to PC2?
Frame
Segment
Packet
L5PDU
L3PDU
L1PDU
Which of the following does a router normally use when making a decision about routing TCP/IP?
Destination MAC address
Source MAC address
Destination IP address
Source IP address
Destination MAC and IP address
Imagine a network with two routers that are connected with a point-to-point HDLC serial link. Each router has an Ethernet, with PC1 sharing the Ethernet with Router1, and PC2 sharing an Ethernet with Router2. When PC1 sends data to PC2, which of the following is true?
Router1 strips the Ethernet header and trailer off the frame received from PC1, never to be used again.
Router1 encapsulates the Ethernet frame inside an HDLC header and sends the frame to Router2, which extracts the Ethernet frame for forwarding to PC2.
Router1 strips the Ethernet header and trailer off the frame received from PC1, which is exactly re-created by R2 before forwarding data to PC2.
Router1 removes the Ethernet, IP, and TCP headers, and rebuilds the appropriate headers before forwarding the packet to Router2.
Which of the following are valid Class C IP addresses?
1.1.1.1
200.1.1.1
128.128.128.128
224.1.1.1
223.223.223.255
What is the range for the values of the first octet for Class A IP networks?
0 to 127
0 to 126
1 to 127
1 to 126
128 to 191
128 to 192
PC1 and PC2 are on two different Ethernets that are separated by an IP router. PC1's IP address is 10.1.1.1, and no subnetting is used. Which of the following addresses could be used for PC2?
10.1.1.2
10.2.2.2
10.200.200.1
9.1.1.1
225.1.1.1
1.1.1.1
How many valid host IP addresses does each Class B network contain?
16,777,214
16,777,216
65,536
65,534
65,532
32,768
32,766
32,764
How many valid host IP addresses does each Class C network contain?
65,536
65,534
65,532
32,768
32,766
256
254
Which of the following protocols allows a client PC to discover the IP address of another computer, based on that other computer's name?
ARP
RARP
DNS
DHCP
BOOTP
Which of the following protocols allow a client PC to request assignment of an IP address as well as learn its default gateway?
ARP
RARP
DNS
DHCP
Which term is defined by the following phrase: "the type of protocol that is being forwarded when routers perform routing."
Routed protocol
Routing protocol
RIP
IOS
Route protocol
The answers to the "Do I Know This Already?" quiz are found in Appendix A, "Answers to the 'Do I Know This Already?' Quizzes and Q&A Sections." The suggested choices for your next step are as follows:
10 or less overall score—Read the entire chapter. This includes the "Foundation Topics" and "Foundation Summary" sections and the "Q&A" section.
11 or 12 overall score—If you want more review on these topics, skip to the "Foundation Summary" section and then go to the "Q&A" section. Otherwise, move to the next chapter.
Foundation Topics
OSI Layer 3–equivalent protocols use routing and addressing to accomplish their goals. The choices made by the people who made up addressing greatly affect how routing works, so the two topics are best described together.
This chapter begins with an overview of the functions of routing and network layer logical addressing. Following that, the text moves on to the basics of IP addressing, relating IP addressing to the OSI routing and addressing concepts covered in the first section. The chapter ends with an introduction to IP routing protocols.