CCENT/CCNA ICND1 Official Exam Certification Guide - Chapter 5 doc

Regardless of the type of physical network to which each endpoint computer is attached, and regardless of the types of physical networks used between the two computers, the network layer

C H A P T E R 5

Fundamentals of IP Addressing and Routing

The OSI physical layer (Layer 1) defines how to transmit bits over a particular type of physical network The OSI data link layer (Layer 2) defines the framing, addressing, error detection, and rules for when to use the physical medium Although they are important, these two layers do not define how to deliver data between devices that exist far from each other, with many different physical networks sitting between the two computers

This chapter explains the function and purpose of the OSI network layer (Layer 3): the end-to-end delivery of data between two computers Regardless of the type of physical network to which each endpoint computer is attached, and regardless of the types of physical networks used between the two computers, the network layer defines how to forward, or route, data between the two computers

This chapter covers the basics of how the network layer routes data packets from one computer to another After reviewing the full story at a basic level, this chapter examines

in more detail the network layer of TCP/IP, including IP addressing (which enables efficient routing), IP routing (the forwarding process itself), IP routing protocols (the process by which routers learn routes), and several other small but important features of the network layer

“Do I Know This Already?” Quiz

The “Do I Know This Already?” quiz allows you to assess whether you should read the entire chapter If you miss no more than one of these 13 self-assessment questions, you might want to move ahead to the “Exam Preparation Tasks” section Table 5-1 lists the major headings in this chapter and the “Do I Know This Already?” quiz questions covering the material in those sections This helps you assess your knowledge of these specific areas The answers to the “Do I Know This Already?” quiz appear in Appendix A

1. Which of the following are functions of OSI Layer 3 protocols?

a. Router1 strips the Ethernet header and trailer off the frame received from PC1, never to be used again

b. Router1 encapsulates the Ethernet frame inside an HDLC header and sends the frame to Router2, which extracts the Ethernet frame for forwarding to PC2

Table 5-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

“Do I Know This Already?” Quiz 95

c. 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

d. Router1 removes the Ethernet, IP, and TCP headers and rebuilds the appropriate headers before forwarding the packet to Router2

4. Which of the following are valid Class C IP addresses that can be assigned to hosts?

6. 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?

7. Each Class B network contains how many IP addresses that can be assigned to hosts?

a. Destination MAC address

b. Source MAC address

c. Destination IP address

d. Source IP address

e. Destination MAC and IP address

10. Which of the following are true about a LAN-connected TCP/IP host and its IP routing (forwarding) choices?

a. The host always sends packets to its default gateway

b. The host sends packets to its default gateway if the destination IP address is in a different class of IP network than the host

c. The host sends packets to its default gateway if the destination IP address is in a different subnet than the host

d. The host sends packets to its default gateway if the destination IP address is in the same subnet as the host

“Do I Know This Already?” Quiz 97

11. Which of the following are functions of a routing protocol?

a. Advertising known routes to neighboring routers

b. Learning routes for subnets directly connected to the router

c. Learning routes, and putting those routes into the routing table, for routes tised to the router by its neighboring routers

adver-d. To forward IP packets based on a packet’s destination IP address

12. Which of the following protocols allows a client PC to discover the IP address of another computer based on that other computer’s name?

Foundation Topics

OSI Layer 3-equivalent protocols define how packets can be delivered from the computer that creates the packet all the way to the computer that needs to receive the packet To reach that goal, an OSI network layer protocol defines the following features:

Routing: The process of forwarding packets (Layer 3 PDUs).

Logical addressing: Addresses that can be used regardless of the type of physical

networks used, providing each device (at least) one address Logical addressingenables the routing process to identify a packet’s source and destination

Routing protocol: A protocol that aids routers by dynamically learning about the

groups of addresses in the network, which in turn allows the routing (forwarding)process to work well

Other utilities: The network layer also relies on other utilities For TCP/IP, these

utilities include Domain Name System (DNS), Dynamic Host Configuration Protocol (DHCP), Address Resolution Protocol (ARP), and ping

This chapter begins with an overview of routing, logical addressing, and routing protocols Following that, the text moves on to more details about the specifics of the TCP/IP network layer (called the internetwork layer in the TCP/IP model) In particular, the topics of IP addressing, routing, routing protocols, and network layer utilities are covered

Overview of Network Layer Functions

A protocol that defines routing and logical addressing is considered to be a network layer,

or Layer 3, protocol OSI does define a unique Layer 3 protocol called Connectionless Network Services (CLNS), but, as usual with OSI protocols, you rarely see it in networks today In the recent past, you might have seen many other network layer protocols, such as Internet Protocol (IP), Novell Internetwork Packet Exchange (IPX), or AppleTalk Datagram Delivery Protocol (DDP) Today, the only Layer 3 protocol that is used widely is the TCP/

IP network layer protocol—specifically, IP

The main job of IP is to route data (packets) from the source host to the destination host Because a network might need to forward large numbers of packets, the IP routing process

is very simple IP does not require any overhead agreements or messages before sending a packet, making IP a connectionless protocol IP tries to deliver each packet, but if a router

or host’s IP process cannot deliver the packet, it is discarded—with no error recovery The

NOTE The term path selection sometimes is used to mean the same thing as routing

protocol, sometimes is used to refer to the routing (forwarding) of packets, and sometimes is used for both functions

Overview of Network Layer Functions 99

goal with IP is to deliver packets with as little per-packet work as possible, which allows for large packet volumes Other protocols perform some of the other useful networking functions For example, Transmission Control Protocol (TCP), which is described in detail

in Chapter 6, “Fundamentals of TCP/IP Transport, Applications, and Security,” provides error recovery, resending lost data, but IP does not

IP routing relies on the structure and meaning of IP addresses, and IP addressing was designed with IP routing in mind This first major section of this chapter begins by introducing IP routing, with some IP addressing concepts introduced along the way Then, the text examines IP addressing fundamentals

Routing (Forwarding)

Routing focuses on the end-to-end logic of forwarding data Figure 5-1 shows a simple example of how routing works The logic illustrated by the figure is relatively simple For PC1 to send data to PC2, it must send something to router R1, which sends it to router R2, and then to router R3, and finally to PC2 However, the logic used by each device along the path varies slightly

Figure 5-1 Routing Logic: PC1 Sending to PC2

to Nearby Router.

My Route

to that Group Is Out Serial Link.

My Route

to that Group Is Out Frame Relay.

PC2

PC1’s Logic: Sending Data to a Nearby Router

In this example, illustrated in Figure 5-1, PC1 has some data to send to PC2 Because PC2

is not on the same Ethernet as PC1, PC1 needs to send the packet to a router that is attached

to the same Ethernet as PC1 The sender sends a data-link frame across the medium to the nearby router; this frame includes the packet in the data portion of the frame That frame uses data link layer (Layer 2) addressing in the data-link header to ensure that the nearby router receives the frame

The main point here is that the computer that created the data does not know much about the network—just how to get the data to some nearby router Using a post office analogy, it’s like knowing how to get to the local post office, but nothing more Likewise, PC1 needs

to know only how to get the packet to R1, not the rest of the path used to send the packet

to PC2

R1 and R2’s Logic: Routing Data Across the Network

R1 and R2 both use the same general process to route the packet The routing table for any particular network layer protocol contains a list of network layer address groupings Instead

of a single entry in the routing table per individual destination network layer address, there

is one routing table entry per group The router compares the destination network layer address in the packet to the entries in the routing table and makes a match This matching entry in the routing table tells this router where to forward the packet next The words in the bubbles in Figure 5-1 point out this basic logic

The concept of network layer address grouping is similar to the U.S zip code system Everyone living in the same vicinity is in the same zip code, and the postal sorters just look for the zip codes, ignoring the rest of the address Likewise, in Figure 5-1, everyone in this network whose IP address starts with 168.1 is on the Ethernet on which PC2 resides, so the routers can have just one routing table entry that means “all addresses that start with 168.1.”

Any intervening routers repeat the same process: the router compares the packet’s destination network layer (Layer 3) address to the groups listed in its routing table, and the matched routing table entry tells this router where to forward the packet next Eventually, the packet is delivered to the router connected to the network or subnet of the destination host (R3), as shown in Figure 5-1

R3’s Logic: Delivering Data to the End Destination

The final router in the path, R3, uses almost the exact same logic as R1 and R2, but with one minor difference R3 needs to forward the packet directly to PC2, not to some other router On the surface, that difference seems insignificant In the next section, when you read about how the network layer uses the data link layer, the significance of the difference will become obvious

Overview of Network Layer Functions 101

Network Layer Interaction with the Data Link Layer

When the network layer protocol is processing the packet, it decides to send the packet out the appropriate network interface Before the actual bits can be placed onto that physical interface, the network layer must hand off the packet to the data link layer protocols, which,

in turn, ask the physical layer to actually send the data And as was described in Chapter 3,

“Fundamentals of LANs,” the data link layer adds the appropriate header and trailer to the packet, creating a frame, before sending the frames over each physical network The routing process forwards the packet, and only the packet, end-to-end through the network,

discarding data-link headers and trailers along the way The network layer processes

deliver the packet end-to-end, using successive data-link headers and trailers just to get the packet to the next router or host in the path Each successive data link layer just gets the packet from one device to the next Figure 5-2 points out the key encapsulation logic on each device, using the same examples as in Figure 5-1

Figure 5-2 Network Layer and Data Link Layer Encapsulation

Extract IP Packet and Encapsulate in HDLC

Extract IP Packet, and Encapsulate in Frame Relay

Extract IP Packet, and Encapsulate in Ethernet

Because the routers build new data-link headers and trailers (trailers not shown in the figure), and because the new headers contain data-link addresses, the PCs and routers must have some way to decide what data-link addresses to use An example of how the router determines which data-link address to use is the IP Address Resolution Protocol (ARP)

ARP is used to dynamically learn the data-link address of an IP host connected to a LAN

You will read more about ARP later in this chapter

Routing as covered so far has two main concepts:

■ The process of routing forwards Layer 3 packets, also called Layer 3 protocol data

units (L3 PDU), based on the destination Layer 3 address in the packet.

■ The routing process uses the data link layer to encapsulate the Layer 3 packets into Layer 2 frames for transmission across each successive data link

IP Packets and the IP Header

The IP packets encapsulated in the data-link frames shown in Figure 5-2 have an IP header, followed by additional headers and data For reference, Figure 5-3 shows the fields inside the standard 20-byte IPv4 header, with no optional IP header fields, as is typically seen in most networks today

Figure 5-3 IPv4 Header

Of the different fields inside the IPv4 header, this book, and the companion ICND2 Official

Exam Certification Guide, ignore all the fields except the Time-To-Live (TTL) (covered in

Chapter 15 in this book), protocol (Chapter 6 of the ICND2 book), and the source and destination IP address fields (scattered throughout most chapters) However, for reference, Table 5-2 briefly describes each field

Version Header

Length

Identification Flags (3) Fragment Offset (13)

Source IP Address

Destination IP Address

Overview of Network Layer Functions 103

This section next examines the concept of network layer addressing and how it aids the routing process

Network Layer (Layer 3) Addressing

Network layer protocols define the format and meaning of logical addresses (The term

logical address does not really refer to whether the addresses make sense, but rather to

contrast these addresses with physical addresses.) Each computer that needs to communicate will have (at least) one network layer address so that other computers can send data packets to that address, expecting the network to deliver the data packet to the correct computer

One key feature of network layer addresses is that they were designed to allow logical grouping of addresses In other words, something about the numeric value of an address implies a group or set of addresses, all of which are considered to be in the same grouping

With IP addresses, this group is called a network or a subnet These groupings work just

like USPS zip (postal) codes, allowing the routers (mail sorters) to speedily route (sort) lots

of packets (letters)

Table 5-2 IPv4 Header Fields

Version Version of the IP protocol Most networks use version 4 today.

IHL IP Header Length Defines the length of the IP header, including optional fields.

DS Field Differentiated Services Field It is used for marking packets for the purpose of

applying different quality-of-service (QoS) levels to different packets.

Packet length Identifies the entire length of the IP packet, including the data.

Identification Used by the IP packet fragmentation process; all fragments of the original

packet contain the same identifier.

Flags 3 bits used by the IP packet fragmentation process.

Fragment offset A number used to help hosts reassemble fragmented packets into the original

larger packet.

TTL Time to live A value used to prevent routing loops.

Protocol A field that identifies the contents of the data portion of the IP packet For example,

protocol 6 implies that a TCP header is the first thing in the IP packet data field.

Header Checksum A value used to store an FCS value, whose purpose is to determine if any bit

errors occurred in the IP header.

Source IP address The 32-bit IP address of the sender of the packet.

Destination IP

address

The 32-bit IP address of the intended recipient of the packet.

Just like postal street addresses, network layer addresses are grouped based on physical location in a network The rules differ for some network layer protocols, but with IP addressing, the first part of the IP address is the same for all the addresses in one grouping For example, in Figures 5-1 and 5-2, the following IP addressing conventions define the groups of IP addresses (IP networks) for all hosts on that internetwork:

■ Hosts on the top Ethernet: Addresses start with 10

■ Hosts on the R1-R2 serial link: Addresses start with 168.10

■ Hosts on the R2-R3 Frame Relay network: Addresses start with 168.11

■ Hosts on the bottom Ethernet: Addresses start with 168.1

Routing relies on the fact that Layer 3 addresses are grouped The routing tables for each network layer protocol can have one entry for the group, not one entry for each individual address Imagine an Ethernet with 100 TCP/IP hosts A router that needs to forward packets

to any of those hosts needs only one entry in its IP routing table, with that one routing table entry representing the entire group of hosts on the Ethernet This basic fact is one of the key reasons that routers can scale to allow hundreds of thousands of devices It’s very similar

to the USPS zip code system It would be ridiculous to have people in the same zip code live far from each other, or to have next-door neighbors be in different zip codes The poor postman would spend all his time driving and flying around the country! Similarly, to make routing more efficient, network layer protocols group addresses

Routing Protocols

Conveniently, the routers in Figures 5-1 and 5-2 somehow know the correct steps to take to forward the packet from PC1 to PC2 To make the correct choices, each router needs a routing table, with a route that matches the packet sent to PC2 The routes tell the router where to send the packet next

In most cases, routers build their routing table entries dynamically using a routing protocol Routing protocols learn about all the locations of the network layer “groups” in a network and advertise the groups’ locations As a result, each router can build a good routing table dynamically Routing protocols define message formats and procedures, just like any other protocol The end goal of each routing protocol is to fill the routing table with all known destination groups and with the best route to reach each group

NOTE To avoid confusion when writing about IP networks, many resources (including

this one) use the term internetwork to refer more generally to a network made up of routers, switches, cables, and other equipment, and the word network to refer to the more

specific concept of an IP network

IP Addressing 105

The terminology relating to routing protocols sometimes can get in the way A routing

protocol learns routes and puts those routes in a routing table A routed protocol defines the

type of packet forwarded, or routed, through a network In Figures 5-1 and 5-2, the figures

represent how IP packets are routed, so IP would be the routed protocol If the routers used Routing Information Protocol (RIP) to learn the routes, RIP would be the routing protocol

Later in this chapter, the section “IP Routing Protocols” shows a detailed example of how routing protocols learn routes

Now that you have seen the basic function of the OSI network layer at work, the rest of this chapter examines the key components of the end-to-end routing process for TCP/IP

IP Addressing

IP addressing is absolutely the most important topic for the CCNA exams By the time you have completed your study, you should be comfortable and confident in your understanding of IP addresses, their formats, the grouping concepts, how to subdivide groups into subnets, how to interpret the documentation for existing networks’ IP addressing, and so on Simply put, you had better know addressing and subnetting!

This section introduces IP addressing and subnetting and also covers the concepts behind the structure of an IP address, including how it relates to IP routing In Chapter 12,

“IP Addressing and Subnetting,” you will read about the math behind IP addressing and subnetting

IP Addressing Definitions

If a device wants to communicate using TCP/IP, it needs an IP address When the device has an IP address and the appropriate software and hardware, it can send and receive

IP packets Any device that can send and receive IP packets is called an IP host.

IP addresses consist of a 32-bit number, usually written in dotted-decimal notation The

“decimal” part of the term comes from the fact that each byte (8 bits) of the 32-bit IP address

is shown as its decimal equivalent The four resulting decimal numbers are written in sequence,

with “dots,” or decimal points, separating the numbers—hence the name dotted decimal For

instance, 168.1.1.1 is an IP address written in dotted-decimal form; the actual binary version is

10101000 00000001 00000001 00000001 (You almost never need to write down the binary version, but you will see how to convert between the two formats in Chapter 12.)

NOTE IP Version 4 (IPv4) is the most widely used version of IP The ICND2 Official

Exam Certification Guide covers the newer version of IP, IPv6 This book only briefly

mentions IPv6 in Chapter 12 and otherwise ignores it So, all references to IP addresses

in this book should be taken to mean “IP version 4” addresses

Each decimal number in an IP address is called an octet The term octet is just a neutral term for byte So, for an IP address of 168.1.1.1, the first octet is 168, the second

vendor-octet is 1, and so on The range of decimal numbers in each vendor-octet is between 0 and 255, inclusive

Finally, note that each network interface uses a unique IP address Most people tend to think that their computer has an IP address, but actually their computer’s network card has an

IP address If you put two Ethernet cards in a PC to forward IP packets through both cards, they both would need unique IP addresses Also, if your laptop has both an Ethernet NIC and a wireless NIC working at the same time, your laptop will have an IP address for each NIC Similarly, routers, which typically have many network interfaces that forward

IP packets, have an IP address for each interface

Now that you have some idea of the basic terminology, the next section relates IP addressing to the routing concepts of OSI Layer 3

How IP Addresses Are Grouped

The original specifications for TCP/IP grouped IP addresses into sets of consecutive

addresses called IP networks The addresses in a single network have the same numeric

value in the first part of all addresses in the network Figure 5-4 shows a simple

internetwork that has three separate IP networks

Figure 5-4 Sample Network Using Class A, B, and C Network Numbers

The conventions of IP addressing and IP address grouping make routing easy For example, all IP addresses that begin with 8 are in the IP network that contains all the hosts on the Ethernet on the left Likewise, all IP addresses that begin with 130.4 are in another IP network that consists of all the hosts on the Ethernet on the right Along the same lines, 199.1.1 is the prefix for all IP addresses on the network that includes the

Network 8.0.0.0 All IP addresses that begin with 8

All IP addresses that begin with 199.1.1

All IP addresses that begin with 130.4

Network 130.4.0.0 Network

199.1.1.0

IP Addressing 107

addresses on the serial link (The only two IP addresses in this last grouping will be the

IP addresses on each of the two routers.) By following this convention, the routers build

a routing table with three entries—one for each prefix, or network number For example, the router on the left can have one route that refers to all addresses that begin with 130.4, with that route directing the router to forward packets to the router on the right

The example indirectly points out a couple of key points about how IP addresses are organized To be a little more explicit, the following two rules summarize the facts about which IP addresses need to be in the same grouping:

■ All IP addresses in the same group must not be separated by a router

■ IP addresses separated by a router must be in different groups

As mentioned earlier in this chapter, IP addressing behaves similarly to zip codes Everyone

in my zip code lives in a little town in Ohio If some members of my zip code were in California, some of my mail might be sent to California by mistake Likewise, IP routing relies on the fact that all IP addresses in the same group (called either a network or a subnet) are in the same general location If some of the IP addresses in my network or subnet were allowed to be on the other side of the internetwork compared to my computer, the routers

in the network might incorrectly send some of the packets sent to my computer to the other side of the network

Classes of Networks

Figure 5-4 and the surrounding text claim that the IP addresses of devices attached to the Ethernet on the left all start with 8 and that the IP addresses of devices attached to the Ethernet on the right all start with 130.4 Why only one number (8) for the “prefix” on the Ethernet on the left and two numbers (130 and 4) on the Ethernet on the right? Well, it all has to do with IP address classes

RFC 791 defines the IP protocol, including several different classes of networks IP defines three different network classes for addresses used by individual hosts—addresses called unicast IP addresses These three network classes are called A, B, and C TCP/IP defines Class D (multicast) addresses and Class E (experimental) addresses as well

By definition, all addresses in the same Class A, B, or C network have the same numeric

value network portion of the addresses The rest of the address is called the host portion of

the address

Using the post office example, the network part of an IP address acts like the zip (postal) code, and the host part acts like the street address Just as a letter-sorting machine three states away from you cares only about the zip code on a letter addressed to you,

a router three hops away from you cares only about the network number that your address resides in.

Class A, B, and C networks each have a different length for the part that identifies the network:

■ Class A networks have a 1-byte-long network part That leaves 3 bytes for the rest of the address, called the host part

■ Class B networks have a 2-byte-long network part, leaving 2 bytes for the host portion

of the address

■ Class C networks have a 3-byte-long network part, leaving only 1 byte for the host part.For example, Figure 5-4 lists network 8.0.0.0 next to the Ethernet on the left Network 8.0.0.0 is a Class A network, which means that only 1 octet (byte) is used for the network part of the address So, all hosts in network 8.0.0.0 begin with 8 Similarly, Class B network 130.4.0.0 is listed next to the Ethernet on the right Because it is a Class B network, 2 octets define the network part, and all addresses begin with 130.4 as the first 2 octets

When listing network numbers, the convention is to write down the network part of the number, with all decimal 0s in the host part of the number So, Class A network “8,” which consists of all IP addresses that begin with 8, is written as 8.0.0.0 Similarly, Class B network “130.4,” which consists of all IP addresses that begin with 130.4, is written as 130.4.0.0, and so on

Now consider the size of each class of network Class A networks need 1 byte for the network part, leaving 3 bytes, or 24 bits, for the host part There are 224 different possible values in the host part of a Class A IP address So, each Class A network can have 224 IP addresses—except for two reserved host addresses in each network, as shown in the last column of Table 5-3 The table summarizes the characteristics of Class A, B, and C networks

* There are two reserved host addresses per network.

Based on the three examples from Figure 5-4, Table 5-4 provides a closer look at the numeric version of the three network numbers: 8.0.0.0, 130.4.0.0, and 199.1.1.0

Table 5-3 Sizes of Network and Host Parts of IP Addresses with No Subnetting

Any Network of This

Class

Number of Network Bytes (Bits)

Number of Host Bytes (Bits)

Number of Addresses Per Network *

Besides the network number, a second dotted-decimal value in each network is reserved

Note that the first reserved value, the network number, has all binary 0s in the host part of the number (see Table 5-4) The other reserved value is the one with all binary 1s in the host

part of the number This number is called the network broadcast or directed broadcast

address This reserved number cannot be assigned to a host for use as an IP address However, packets sent to a network broadcast address are forwarded to all devices in the network

Also, because the network number is the lowest numeric value inside that network and the broadcast address is the highest numeric value, all the numbers between the network number and the broadcast address are the valid, useful IP addresses that can be used to address interfaces in the network

The Actual Class A, B, and C Network Numbers

The Internet is a collection of almost every IP-based network and almost every TCP/IP host computer in the world The original design of the Internet required several cooperating features that made it technically possible as well as administratively manageable:

■ Each computer connected to the Internet needs a unique, nonduplicated IP address

■ Administratively, a central authority assigned Class A, B, or C networks to companies, governments, school systems, and ISPs based on the size of their IP network (Class A for large networks, Class B for medium networks, and Class C for small networks)

■ The central authority assigned each network number to only one organization, helping ensure unique address assignment worldwide

■ Each organization with an assigned Class A, B, or C network then assigned individual

IP addresses inside its own network

Table 5-4 Sample Network Numbers, Decimal and Binary

Network Number Binary Representation, with the Host Part in Bold

8.0.0.0 00001000 00000000 00000000 00000000

130.4.0.0 10000010 00000100 00000000 00000000

199.1.1.0 11000111 00000001 00000001 00000000

By following these guidelines, as long as each organization assigns each IP address to only one computer, every computer in the Internet has a globally unique IP address.

The organization in charge of universal IP address assignment is the Internet Corporation for Assigned Network Numbers (ICANN, www.icann.org) (The Internet Assigned Numbers Authority (IANA) formerly owned the IP address assignment process.) ICANN, in turn, assigns regional authority to other cooperating organizations For example, the American Registry for Internet Numbers (ARIN, www.arin.org) owns the address assignment process for North America

Table 5-5 summarizes the possible network numbers that ICANN and other agencies could have assigned over time Note the total number for each network class and the number

of hosts in each Class A, B, and C network

* The Valid Network Numbers column shows actual network numbers Networks 0.0.0.0 (originally defined for use as

a broadcast address) and 127.0.0.0 (still available for use as the loopback address) are reserved.

Memorizing the contents of Table 5-5 should be one of the first things you do in preparation for the CCNA exam(s) Engineers should be able to categorize a network as Class A, B,

or C with ease Also, memorize the number of octets in the network part of Class A, B, and

C addresses, as shown in Table 5-4

Valid Network Numbers *

Total Number for This Class of Network

Number of Hosts Per Network