IPv4 Addressing > CCNP 1: Advanced IP Addressing Management | Cisco Press

IPv4 Addressing

This section covers some of the basic concepts of IPv4 addressing, such as
how the Internet’s address architecture uses the binary and dotted-decimal
versions of IPv4 addressing. This section also reviews the structure of IPv4
addresses, such as the various classes of IPv4 addresses. Finally, this section
reviews how IPv4 addresses use subnet masks to help divide and manage the size
and growth of the Internet and computer networks.

Address Architecture of the Internet

When TCP/IP was introduced in the 1980s, it relied on a two-level addressing
scheme. At the time, this scheme offered adequate scalability. The 32-bit-long
IPv4 address identifies a network number and a host number, as shown in Figure
2-1.

Figure
2-1 IP Address Structure

Together, the network number and the host number uniquely identify all hosts
connected by way of the Internet. It is possible that the needs of a small
networked community, such as a LAN, could be satisfied with just host addresses.
However, network addresses are necessary for end systems on different networks
to communicate with each other. Routers use the network portion of the address
to make routing decisions and to facilitate communication between hosts that
belong to different networks.

Unlike routers, humans find working with strings of 32 1s and 0s tedious and
clumsy. Therefore, 32-bit IP addresses are written using dotted-decimal
notation. Each 32-bit address is divided into four groups of eight, called
octets. Each octet is converted to decimal and then separated by decimal points,
or dots. This is illustrated as follows:

• A 32-bit IP address is a binary number:

10101100000111101000000000010001

10101100 00011110 10000000 00010001

172 30 128 17

172.30.128.17

In the dotted-decimal address 172.30.128.17, which of these four numbers
represents the network portion of the address? Which numbers are the host
numbers? Finding the answers to these questions is complicated by the fact that
IP addresses are not really four numbers. They actually consist of 32 different
numbers, or 32 bits.

In the early days of TCP/IP, a class system was used to define the network
and host portions of the address. IPv4 addresses were grouped into five distinct
classes. This was done according to the value of the first few bits in the first
octet of the address. Although the class system can still be applied to IP
addresses, networks today often ignore the rules of class in favor of a
classless IP scheme.

The next few sections cover all of the following topics related to IP
addressing:

• The limitations of the IP address classes

• The subsequent addition of the subnet mask

• The addressing crisis that led to the adoption of a classless
  system

Class A and B IP Addresses

In a class system, IP addresses can be grouped into one of five different
classes:

• A

• B

• C

• D

• E

Each of the four octets of an IP address represents either the network
portion or the host portion of the address, depending on the address class. The
network and host portions of the respective Class A, B, C, and D addresses are
shown in Figure 2-2.

Figure
2-2 Address Structure

Only the first three classes—A, B, and C—are used to address actual
hosts on IP networks. Class D addresses are used for multicasting. Class E
addresses are reserved for experimentation and are not shown in Figure 2-2. The
following sections explore each of the five classes of addresses.

Class A Addresses

If the first bit of the first octet of an IP address is a binary 0, the
address is a Class A address. With that first bit being a 0, the lowest number
that can be represented is 00000000, decimal 0. The highest number that can be
represented is 01111111, decimal 127. Any address that starts with a value
between 0 and 127 in the first octet is a Class A address. These two numbers, 0
and 127, are reserved and cannot be used as a network address.

Class A addresses were intended to accommodate very large networks, so only
the first octet is used to represent the network number. This leaves three
octets, or 24 bits, to represent the host portion of the address. With 24 bits
total, 224 combinations are possible, yielding 16,777,216 possible
addresses. Two of those possibilities, the lowest and highest values, are
reserved for special purposes. The low value is 24 0s, and the high value is 24
1s. Therefore, each Class A address can support up to 16,777,214 unique host
addresses.

Why are two host addresses reserved for special purposes? Every network
requires a network number. A network number is an ID number that is used to
refer to the entire range of hosts when building routing tables. The address
that contains all 0s in the host portion is used as the network number and
cannot be used to address an individual node. 46.0.0.0 is a Class A network
number. Similarly, every network requires a broadcast address that can be used
to address a message to every host on a network. It is created when the host
portion of the address has all 1s. For example, a broadcast address for network
46.0.0.0 would be 46.255.255.255.

With almost 17 million host addresses available, a Class A network actually
provides too many possibilities for one company or campus. Although it is easy
to imagine an enormous global network with that many nodes, the hosts in such a
network could not function as members of the same logical group. Administrators
require much smaller logical groupings to control broadcasts, apply policies,
and troubleshoot problems. Fortunately, the subnet mask allows subnetting, which
breaks a large block of addresses into smaller groups called subnetworks. All
Class A networks are subnetted. If they were not, Class A networks would
represent huge waste and inefficiency.

How many Class A addresses are there? Because only the first octet is used as
a network number, and it contains a value between 0 and 126, 126 Class A
networks exist. Each of the 126 Class A addresses has almost 17 million possible
host addresses that make up about half of the entire IPv4 address space. Recall
that the network address 127.0.0.1 is reserved for the local loopback address,
which is why Class A addresses stop at 126.0.0.0 and Class B addresses start at
128.0.0.0. Under this system, a mere handful of organizations control half of
the available Internet addresses.

Class B Addresses

Class B addresses start with a binary 10 in the first 2 bits of the first
octet. Therefore, the lowest number that can be represented with a Class B
address is 10000000, decimal 128. The highest number that can be represented is
10111111, decimal 191. Any address that starts with a value in the range of 128
to 191 in the first octet is a Class B address.

Class B addresses were intended to accommodate medium-size networks.
Therefore, the first two octets are used to represent the network number, which
leaves two octets or 16 bits to represent the host portion of the address. With
16 bits total, 216 combinations are possible, yielding 65,536 Class B
addresses. Recall that two of those numbers, the lowest and highest values, are
reserved for special purposes. Therefore, each Class B address can support up to
65,534 hosts. Although it is significantly smaller than the networks created by
Class A addresses, a logical group of more than 65,000 hosts is still
unmanageable and impractical. Therefore, like Class A networks, Class B
addresses are subnetted to improve efficiency.

Because the first 2 bits of a Class B address are always 10, 14 bits are left
in the network portion of the address, resulting in 214 or 16,384
Class B networks. The first octet of a Class B address offers 64 possibilities,
128 to 191. The second octet has 256 possibilities, 0 to 255. That yields 16,384
addresses, or 25 percent of the total IP space. Nevertheless, given the
popularity and importance of the Internet, these addresses have run out quickly.
This essentially leaves only Class C addresses available for new growth.

Classes of IP Addresses: C, D, and E

This section covers Class C, D, and E IP addresses.

Class C Addresses

A Class C address begins with binary 110. Therefore, the lowest number that
can be represented is 11000000, decimal 192. The highest number that can be
represented is 11011111, decimal 223. If an IPv4 address contains a number in
the range of 192 to 223 in the first octet, it is a Class C address.

Class C addresses were originally intended to support small networks. The
first three octets of a Class C address represent the network number. The last
octet may be used for hosts. One host octet yields 256 (28)
possibilities. After the all-0s network number and the all-1s broadcast address
are subtracted, only 254 hosts may be addressed on a Class C network. Whereas
Class A and Class B networks prove impossibly large without subnetting, Class C
networks can impose an overly restrictive limit on hosts.

Because the first 3 bits of a Class C address are always 110, 21 bits are
left in the network portion of the address, resulting in 221 or
2,097,152 Class C networks. With 2,097,152 total network addresses containing a
mere 254 hosts each, Class C addresses account for 12.5 percent of the Internet
address space. Because Class A and B addresses are nearly exhausted, the
remaining Class C addresses are all that is left to be assigned to new
organizations that need IP networks. Table 2-1 summarizes the ranges and
availability of the three address classes used to address Internet hosts.

Table 2-1 IP Addresses Available to Internet Hosts

Address Class

First Octet Range

Number of Possible Networks

Number of Hosts Per Network

Class A

0 to 126

127 (2 are reserved)

16,777,214

Class B

128 to 191

16,384

65,534

Class C

192 to 223

2,097,152

254

Class D Addresses

A Class D address begins with binary 1110 in the first octet. Therefore, the
first octet range for a Class D address is 11100000 to 11101111, or 224 to 239.
Class D addresses are not used to address individual hosts. Instead, each Class
D address can be used to represent a group of hosts called a host group, or
multicast group.

For example, a router configured to run Enhanced Interior Gateway Routing
Protocol (EIGRP) joins a group that includes other nodes that are also running
EIGRP. Members of this group still have unique IP addresses from the Class A, B,
or C range, but they also listen for messages addressed to 224.0.0.10. The 224
octet designates the address as a Class D address. Therefore, a single routing
update message can be sent to 224.0.0.10, and all EIGRP routers will receive it.
A single message sent to several select recipients is called a multicast. Class
D addresses are also called multicast addresses.

A multicast is different from a broadcast. Every device on a logical network
must process a broadcast, whereas only devices configured to listen for a Class
D address receive a multicast.

Class E Addresses

If the first octet of an IP address begins with 1111, the address is a Class
E address. Therefore, the first octet range for Class E addresses is 11110000 to
1111111, or 240 to 255. Class E addresses are reserved for experimental purposes
and should not be used to address hosts or multicast groups.

Subnet Masking

Subnet masking, or subnetting, is used to break one large group into several
smaller subnetworks, as shown in Figure 2-3. These subnets can then be
distributed throughout an enterprise. This results in less IP address waste and
better logical organization. Formalized with RFC 950 in 1985, subnetting
introduced a third level of hierarchy to the IPv4 addressing structure. The
number of bits available to the network, subnet, and host portions of a given
address varies depending on the size of the subnet mask.

Figure
2-3 IP Address Structure After Subnetting

A subnet mask is a 32-bit number that acts as a counterpart to the IP
address. Each bit in the mask corresponds to its counterpart bit in the IP
address. Logical ANDing is applied to the address and mask. If a bit in the IP
address corresponds to a 1 bit in the subnet mask, the IP address bit represents
a network number. If a bit in the IP address corresponds to a 0 bit in the
subnet mask, the IP address bit represents a host number.

When the subnet mask is known, it overrides the address class to determine
whether a bit is either a network or a host. This allows routers to recognize
addresses differently than the format dictated by class. The mask can be used to
tell hosts that although their addresses are Class B, the first three octets,
instead of the first two, are the network number. In this case, the additional
octet acts like part of the network number, but only inside the organization
where the mask is configured.

The subnet mask applied to an address ultimately determines the network and
host portions of an IP address. The network and host portions change when the
subnet mask changes. If a 16-bit mask, 255.255.0.0, is applied to an IP address,
only the first 16 bits, or two octets, of the IP address 172.24.100.45 represent
the network number. Therefore, the network number for this host address is
172.24.0.0. The colored portion of the address shown in Figure 2-4 indicates the
network number.

Figure
2-4
Class B Address Without Subnetting

Because the rules of class dictate
that the first two octets of a Class B address are the network number, this
16-bit mask does not create subnets within the 172.24.0.0 network.

To create subnets with this Class B address, a mask must be used that
identifies bits in the third or fourth octet as part of the network number.

If a 24-bit mask such as 255.255.255.0 is applied, the first 24 bits of the
IP address are specified as the network number. The network number for the host
in this example is 172.24.100.0. The gray portion of the address shown in Figure
2-5 indicates this.

Routers and hosts configured with this mask see all 8 bits in the third octet
as part of the network number. These 8 bits are considered to be the subnet
field because they represent network bits beyond the two octets prescribed by
classful addressing.

Inside this network, devices configured with a 24-bit mask use the 8 bits of
the third octet to determine to what subnet a host belongs. Because 8 bits
remain in the host field, 254 hosts may populate each network. Just as hosts
must have identical network addresses, they also must match subnet fields to
communicate with each other directly. Otherwise, the services of a router must
be used so that a host on one network or subnet can talk to a host on
another.

Figure
2-5 Class B Address with Subnetting

A Class B network with an 8-bit subnet field creates 28, or 256,
potential subnets, each one equivalent to one Class C network. Because 8 bits
remain in the host field, 254 hosts may populate each network. Two host
addresses are reserved as the network number and broadcast address,
respectively. By dividing a Class B network into smaller logical groups, the
internetwork can be made more manageable, more efficient, and more scalable.

Notice that subnet masks are not sent as part of an IP packet header. This
means that routers outside this network will not know what subnet mask is
configured inside the network. An outside router, therefore, treats
172.24.100.45 as just one of 65,000 hosts that belong to the 172.24.0.0 network.
In effect, subnetting classful IP addresses provides a logical structure that is
hidden from the outside world.

Interactive Media Activity Fill in the Blank:
Subnet Tool

After completing this activity, you will have a better understanding of the
concept of subnetting.