CCNA1 6 Addressing the Network - IPv4

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CCNA Exploration - Network Fundamentals

6 Addressing the Network - IPv4

6.0 Chapter Introduction

6.0.1 Chapter Introduction

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Addressing is a key function of Network layer protocols that enables data communication between hosts on the same network or on different networks. Internet Protocol version 4 (IPv4) provides hierarchical addressing for packets that carry our data.

Designing, implementing and managing an effective IPv4 addressing plan ensures that our networks can operate effectively and efficiently.

This chapter examines in detail the structure of IPv4 addresses and their application to the construction and testing of IP networks and subnetworks.

In this chapter, you will learn to:

Explain the structure IP addressing and demonstrate the ability to convert between 8-bit binary and decimal numbers.

Given an IPv4 address, classify by type and describe how it is used in the network.

Explain how addresses are assigned to networks by ISPs and within networks by administrators.

Determine the network portion of the host address and explain the role of the subnet mask in dividing networks.

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Given IPv4 addressing information and design criteria, calculate the appropriate addressing components.

Use common testing utilities to verify and test network

connectivity and operational status of the IP protocol stack on a host.

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6.1 IPv4 Addresses

6.1.1 The Anatomy of an IPv4 Address

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Each device on a network must be uniquely defined. At the Network layer, the packets of the communication need to be identified with the source and destination addresses of the two end systems. With IPv4, this means that each packet has a 32-bit source address and a 32-bit destination address in the Layer 3 header.

These addresses are used in the data network as binary patterns. Inside the devices, digital logic is applied for their interpretation. For us in the human network, a string of 32 bits is difficult to interpret and even more difficult to remember. Therefore, we represent IPv4 addresses using dotted decimal format.

Dotted Decimal

Binary patterns representing IPv4 addresses are expressed as dotted decimals by separating each byte of the binary pattern, called an octet, with a dot. It is called an octet because each decimal number represents one byte or 8 bits.

For example, the address:

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10101100000100000000010000010100

is expressed in dotted decimal as:

172.16.4.20

Keep in mind that devices use binary logic. The dotted decimal format is used to make it easier for people to use and remember addresses.

Network and Host Portions

For each IPv4 address, some portion of the high-order bits represents the network address. At Layer 3, we define a network as a group of hosts that have identical bit patterns in the network address portion of their addresses.

Although all 32 bits define the IPv4 host address, we have a variable number of bits that are called the host portion of the address. The number of bits used in this host portion determines the number of hosts that we can have within the network.

Click the labels in the figure to see the different parts of the address.

For example, if we need to have at least 200 hosts in a particular network, we would need to use enough bits in the host portion to be able to represent at least 200 different bit patterns.

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To assign a unique address to 200 hosts, we would use the entire last octet. With 8 bits, a total of 256 different bit patterns can be achieved. This would mean that the bits for the upper three octets would represent the network portion.

Note: Calculating the number of hosts and determining which portion of the 32 bits refers to the network will be covered later in this chapter. Display Media Text Transcript | Display Visual Media

6.1.2 Knowing the Numbers - Binary to Decimal Conversion

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To understand the operation of a device in a network, we need to look at addresses and other data the way the device does - in binary notation. This means that we need to have some skill in binary to decimal conversion.

Data represented in binary may represent many different forms of data to the human network. In this discussion, we refer to binary as it relates to IPv4 addressing. This means that we look at each byte (octet) as a decimal number in the range of 0 to 255.

Positional Notation

Learning to convert binary to decimal requires an understanding of the mathematical basis of a numbering system called positional notation. Positional notation means that a digit represents different values depending on the position it occupies. More specifically, the value that a digit represents is that value multiplied by the power of the base, or radix, represented by the position the digit occupies. Some examples will help to clarify how this system works.

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For the decimal number 245, the value that the 2 represents is 2*10^2 (2 times 10 to the power of 2). The 2 is in what we commonly refer to as the "100s" position. Positional notation refers to this position as the base^2 position because the base, or radix, is 10 and the power is 2.

Using positional notation in the base 10 number system, 245 represents:

245 = (2 * 10^2) + (4 * 10^1) + (5 * 10^0)

or

245 = (2 * 100) + (4 * 10) + (5 * 1)

Binary Numbering System

In the binary numbering system, the radix is 2. Therefore, each position represents increasing powers of 2. In 8-bit binary numbers, the positions represent these quantities:

2^7 2^6 2^5 2^4 2^32^2 2^1 2^0

128 64 32 16 8 4 2 1

The base 2 numbering system only has two digits: 0 and 1.

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When we interpret a byte as a decimal number, we have the quantity that position represents if the digit is a 1 and we do not have that quantity if the digit is a 0, as shown in the figure.

1 1 1 1 1 1 1 1

128 64 32 16 8 4 2 1

A 1 in each position means that we add the value for that position to the total. This is the addition when there is a 1 in each position of an octet. The total is 255.

128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255

A 0 in each position indicates that the value for that position is not added to the total. A 0 in every position yields a total of 0.

0 0 0 0 0 0 0 0

128 64 32 16 8 4 2 1

0 + 0 + 0 + 0 + 0 + 0 + 0 + 0 = 0

Notice in the figure that a different combination of ones and zeros will yield a different decimal value. Display Media Text Transcript | Display Visual Media

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See the figure for the steps to convert a binary address to a decimal address.

In the example, the binary number:

10101100000100000000010000010100

Is converted to:

172.16.4.20

Keep these steps in mind:

Divide the 32 bits into 4 octets.

Convert each octet to decimal.

Add a "dot" between each decimal.

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6.1.3 Practicing Binary to Decimal Conversions

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The activity in the figure allows you to practice 8-bit binary conversion as much as necessary. We recommend that you work with this tool until you are able to do the conversion without error. Display Media Text Transcript | Display Visual Media

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6.1.4 Knowing the Numbers - Decimal to Binary Conversions

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Not only do we need to be able to convert binary to decimal, we also need to be able to convert decimal to binary. We often need to examine an individual octet of an address that is given in dotted decimal notation. Such is the case when the network bits and host bits divide an octet.

As an example, if a host with the 172.16.4.20 were using 28 bits for the network address, we would need to examine the binary in the last octet to discover that this host is on the network 172.16.4.16. This process of extracting the network address from a host address will be explained later.

Address Values are Between 0 and 255

Because our representation of addresses is limited to decimal values for a single octet, we will only examine the process of converting 8-bit binary to the decimal values of 0 to 255.

To begin the conversion process, we start by determining if the decimal number is equal to or greater than our largest decimal value represented by the most-significant bit. In the highest position, we determine if the value is equal to or greater than 128. If the value is smaller than 128, we place a 0 in the 128-bit position and move to the 64-bit position.

If the value in the 128-bit position is larger than or equal to 128, we place a 1 in the 128 position and subtract 128 from the number being converted. We then compare the remainder of this operation to the next smaller value, 64. We continue this process for all the remaining bit positions.

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See the figure for an example of these steps. We convert 172 to 10101100. Display Media Text Transcript | Display Visual Media

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Follow the conversion steps to see how an IP address is converted to binary. Display Media Text Transcript | Display Visual Media

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Conversion Summary

The figure summarizes the entire conversion of 172.16.4.20 from dotted decimal notation to binary notation. Display Media Text Transcript | Display Visual Media

6.1.5 Practicing Decimal to Binary Conversion

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The activity in the figure allows you to practice decimal conversion to 8-bit binary as much as necessary. We recommend that you work with this tool until you are able to do the conversion without error. Display Media Text Transcript | Display Visual Media

6.2 Addresses for Different Purposes

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6.2.1 Types of Addresses in an IPv4 Network

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Within the address range of each IPv4 network, we have three types of addresses:

Network address - The address by which we refer to the network

Broadcast address - A special address used to send data to all hosts in the network

Host addresses - The addresses assigned to the end devices in the network

Network Address

The network address is a standard way to refer to a network. For example, we could refer to the network shown in the figure as "the 10.0.0.0 network." This is a much more convenient and descriptive way to refer to the network than using a term like "the first network." All hosts in the 10.0.0.0 network will have the same network bits.

Within the IPv4 address range of a network, the lowest address is reserved for the network address. This address has a 0 for each host bit in the host portion of the address.

Roll over the NETWORK ADDRESS tab in the figure.

Broadcast Address

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The IPv4 broadcast address is a special address for each network that allows communication to all the hosts in that network. To send data to all hosts in a network, a host can send a single packet that is addressed to the broadcast address of the network.

The broadcast address uses the highest address in the network range. This is the address in which the bits in the host portion are all 1s. For the network 10.0.0.0 with 24 network bits, the broadcast address would be 10.0.0.255. This address is also referred to as the directed broadcast.

Roll over the BROADCAST ADDRESS tab in the figure.

Host Addresses

As described previously, every end device requires a unique address to deliver a packet to that host. In IPv4 addresses, we assign the values between the network address and the broadcast address to the devices in that network.

Roll over the HOST ADDRESS tab in the figure. Display Media Text Transcript | Display Visual Media

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Network Prefixes

An important question is: How do we know how many bits represent the network portion and how many bits represent the host portion? When we express an IPv4 network address, we add a prefix length to the network

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address. The prefix length is the number of bits in the address that gives us the network portion. For example, in 172.16.4.0 /24, the /24 is the prefix length - it tells us that the first 24 bits are the network address. This leaves the remaining 8 bits, the last octet, as the host portion. Later in this chapter, we will learn more about another entity that is used to specify the network portion of an IPv4 address to the network devices. It is called the subnet mask. The subnet mask consists of 32 bits, just as the address does, and uses 1s and 0s to indicate which bits of the address are network bits and which bits are host bits.

Networks are not always assigned a /24 prefix. Depending on the number of hosts on the network, the prefix assigned may be different. Having a different prefix number changes the host range and broadcast address for each network.

Roll over the addresses in the figure to view the results of using different prefixes on an address.

Notice that the network address could remain the same, but the host range and the broadcast address are different for the different prefix lengths. In this figure you can also see that the number of hosts that can be addressed on the network changes as well. Display Media Text Transcript | Display Visual Media

6.2.2 Calculating Network, Hosts and Broadcast Addresses

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At this point, you may be wondering: How do we calculate these addresses? This calculation process requires us to look at these addresses in binary.

In the example network divisions, we need to look at the octet of the address where the prefix divides the network portion from the host portion.

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In all of these examples, it is the last octet. While this is common, the prefix can also divide any of the octets.

To get started understanding this process of determining the address assignments, let's break some examples down into binary.

See the figure for an example of the address assignment for the 172.16.20.0 /25 network.

In the first box, we see the representation of the network address. With a 25 bit prefix, the last 7 bits are host bits. To represent the network address, all of these host bits are '0'. This makes the last octet of the address 0. This makes the network address 172.16.20.0 /25.

In the second box, we see the calculation of the lowest host address. This is always one greater than the network address. In this case, the last of the seven host bits becomes a '1'. With the lowest bit of host address set to a 1, the lowest host address is 172.16.20.1.

The third box shows the calculation of the broadcast address of the network. Therefore, all seven host bits used in this network are all '1s'. From the calculation, we get 127 in the last octet. This gives us a broadcast address of 172.16.20.127.

The fourth box presents the calculation of the highest host address. The highest host address for a network is always one less than the broadcast. This means the lowest host bit is a '0' and all other host bits as '1s'. As seen, this makes the highest host address in this network

172.16.20.126.

Although for this example we expanded all of the octets, we only need to examine the content of the divided octet.

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Display Media Text Transcript | Display Visual Media

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Flash Practice Activity

In the activity in the figure, you will calculate the network address, host addresses, and broadcast address for given networks. Practice as much as necessary. We recommend that you work with this tool until you are able to do the conversion without error. Display Media Text Transcript | Display Visual Media

6.2.3 Unicast, Broadcast, Multicast - Types of Communication

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In an IPv4 network, the hosts can communicate one of three different ways:

Unicast - the process of sending a packet from one host to an individual host

Broadcast - the process of sending a packet from one host to all hosts in the network

Multicast - the process of sending a packet from one host to a selected group of hosts

These three types of communication are used for different purposes in the data networks. In all three cases, the IPv4 address of the originating host is placed in the packet header as the source address.

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Unicast Traffic

Unicast communication is used for the normal host-to-host communication in both a client/server and a peer-to-peer network. Unicast packets use the host address of the destination device as the destination address and can be routed through an internetwork. Broadcast and multicast, however, use special addresses as the destination address. Using these special addresses, broadcasts are generally restricted to the local network. The scope of multicast traffic also may be limited to the local network or routed through an internetwork.

Play the animation to see an example of unicast transmission.

In an IPv4 network, the unicast address applied to an end device is referred to as the host address. For unicast communication, the host addresses assigned to the two end devices are used as the source and destination IPv4 addresses. During the encapsulation process, the source host places its IPv4 address in the unicast packet header as the source host address and the IPv4 address of the destination host in the packet header as the destination address. The communication using a unicast packet can be forwarded through an internetwork using the same addresses.

Note: In this course, all communications between devices is unicast communication unless otherwise noted. Display Media Text Transcript | Display Visual Media

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Broadcast Transmission

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Because broadcast traffic is used to send packets to all hosts in the network, a packet uses a special broadcast address. When a host receives a packet with the broadcast address as the destination, it processes the packet as it would a packet to its unicast address.

Broadcast transmission is used for the location of special

services/devices for which the address is not known or when a host needs to provide information to all the hosts on the network.

Some examples for using broadcast transmission are:

Mapping upper layer addresses to lower layer addresses

Requesting an address

Exchanging routing information by routing protocols

When a host needs information, the host sends a request, called a query, to the broadcast address. All hosts in the network receive and process this query. One or more of the hosts with the requested information will respond, typically using unicast.

Similarly, when a host needs to send information to the hosts on a network, it creates and sends a broadcast packet with the information.

Unlike unicast, where the packets can be routed throughout the

internetwork, broadcast packets are usually restricted to the local network. This restriction is dependent on the configuration of the router that borders the network and the type of broadcast. There are two types of broadcasts: directed broadcast and limited broadcast.

Directed Broadcast

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A directed broadcast is sent to all hosts on a specific network. This type of broadcast is useful for sending a broadcast to all hosts on a non-local network. For example, for a host outside of the network to communicate with the hosts within the 172.16.4.0 /24 network, the

destination address of the packet would be 172.16.4.255. This is shown in the figure. Although routers do not forward directed broadcasts by default, they may be configured to do so.

Limited Broadcast

The limited broadcast is used for communication that is limited to the hosts on the local network. These packets use a destination IPv4 address 255.255.255.255. Routers do not forward this broadcast. Packets addressed to the limited broadcast address will only appear on the local network. For this reason, an IPv4 network is also referred to as a broadcast domain. Routers form the boundary for a broadcast domain.

As an example, a host within the 172.16.4.0 /24 network would broadcast to all the hosts in its network using a packet with a destination address of 255.255.255.255.

Play the animation to see an example of broadcast transmission.

As you learned earlier, when a packet is broadcast, it uses resources on the network and also forces every host on the network that receives it to process the packet. Therefore, broadcast traffic should be limited so that it does not adversely affect performance of the network or devices. Because routers separate broadcast domains, subdividing networks with excessive broadcast traffic can improve network performance. Display Media Text Transcript | Display Visual Media

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Multicast Transmission

Multicast transmission is designed to conserve the bandwidth of the IPv4 network. It reduces traffic by allowing a host to send a single packet to a selected set of hosts. To reach multiple destination hosts using unicast communication, a source host would need to send an individual packet addressed to each host. With multicast, the source host can send a single packet that can reach thousands of destination hosts.

Some examples of multicast transmission are:

Video and audio distribution

Routing information exchange by routing protocols

Distribution of software

News feeds

Multicast Clients

Hosts that wish to receive particular multicast data are called multicast clients. The multicast clients use services initiated by a client program to subscribe to the multicast group.

Each multicast group is represented by a single IPv4 multicast destination address. When an IPv4 host subscribes to a multicast group, the host processes packets addressed to this multicast address as well as packets addressed to its uniquely allocated unicast address. As we will see, IPv4 has set aside a special block of addresses from 224.0.0.0 to

239.255.255.255 for multicast groups addressing.

The animation demonstrates clients accepting multicast packets. Display Media Text Transcript | Display Visual Media

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In this activity, you will be able to visualize unicasts, broadcasts, and multicasts by using Packet Tracer in simulation mode.

Broadcast: /rfc/rfc0919.txt?number=919

Multicast: /en/US/tech/tk828/technologies_white_paper09186a0080092942.shtml

/en/US/docs/internetworking/technology/handbook/IP-Multi.html

Click the Packet Tracer icon to launch the Packet Tracer activity. Display Media Text Transcript | Display Visual Media

6.2.4 Reserved IPv4 Address Ranges

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Expressed in dotted decimal format, the IPv4 address range is 0.0.0.0 to 255.255.255.255. As you have already seen, not all of these addresses can be used as host addresses for unicast communication.

Experimental Addresses

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One major block of addresses reserved for special purposes is the IPv4 experimental address range 240.0.0.0 to 255.255.255.254. Currently, these addresses are listed as reserved for future use (RFC 3330). This suggests that they could be converted to usable addresses. Currently, they cannot be used in IPv4 networks. However, these addresses could be used for research or experimentation.

Multicast Addresses

As previously shown, another major block of addresses reserved for special purposes is the IPv4 multicast address range 224.0.0.0 to 239.255.255.255. Additionally, the multicast address range is subdivided into different types of addresses: reserved link local addresses and globally scoped addresses. One additional type of multicast address is the

administratively scoped addresses, also called limited scope addresses.

The IPv4 multicast addresses 224.0.0.0 to 224.0.0.255 are reserved link local addresses. These addresses are to be used for multicast groups on a local network. Packets to these destinations are always transmitted with a time-to-live (TTL) value of 1. Therefore, a router connected to the local network should never forward them. A typical use of reserved link-local addresses is in routing protocols using multicast transmission to exchange routing information.

The globally scoped addresses are 224.0.1.0 to 238.255.255.255. They may be used to multicast data across the Internet. For example, 224.0.1.1 has been reserved for Network Time Protocol (NTP) to synchronize the time-of-day clocks of network devices.

Host Addresses

After accounting for the ranges reserved for experimental addresses and multicast addresses, this leaves an address range of 0.0.0.0 to

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223.255.255.255 that could be used for IPv4 hosts. However, within this range are many addresses that are already reserved for special purposes. Although we have previously covered some of these addresses, the major reserved addresses are discussed in the next section. Display Media Text Transcript | Display Visual Media

6.2.5 Public and Private Addresses

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Although most IPv4 host addresses are public addresses designated for use in networks that are accessible on the Internet, there are blocks of addresses that are used in networks that require limited or no Internet access. These addresses are called private addresses.

Private Addresses

The private address blocks are:

10.0.0.0 to 10.255.255.255 (10.0.0.0 /8)

172.16.0.0 to 172.31.255.255 (172.16.0.0 /12)

192.168.0.0 to 192.168.255.255 (192.168.0.0 /16)

Private space address blocks, as shown in the figure, are set aside for use in private networks. The use of these addresses need not be unique among outside networks. Hosts that do not require access to the Internet at large may make unrestricted use of private addresses. However, the internal networks still must design network address schemes to ensure that the hosts in the private networks use IP addresses that are unique within their networking environment.

Many hosts in different networks may use the same private space addresses. Packets using these addresses as the source or destination should not

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