Understanding IP Addressing: Everything You Ever Wanted To Know

Classful IP Addressing

When IP was first standardized in September 1981, the specification required that each system attached to an IP-based internet be assigned a unique, 32-bit Internet address value. Some systems, such as routers which have interfaces to more than one network, must be assigned a unique IP address for each network interface.
The first part of an Internet address identifies the network on which the host resides, while the second part identifies the particular host on the given network. This created the two-level addressing hierarchy which is illustrated in Figure 3.

Figure 3: Two-Level Internet Address Structure

In recent years, the network-number field has been referred to as the “network-prefix” because the leading portion of each IP address identifies the network number. All hosts on a given network share the same network-prefix but must have a unique host-number. Similarly, any two hosts on different networks must have different network-prefixes but may have the same host-number.

Primary Address Classes
In order to provide the flexibility required to support different size networks, the designers decided that the IP address space should be divided into three different address classes – Class A, Class B, and Class C. This is often referred to as “classful” addressing because the address space is split into three predefined classes, groupings, or categories. Each class fixes the boundary between the network-prefix and the host-number at a different point within the 32-bit address. The formats of the fundamental address classes are illustrated in Figure 4.

Figure 4: Principle Classful IP Address Formats

One of the fundamental features of classful IP addressing is that each address contains a self-encoding key that identifies the dividing point between the network-prefix and the host-number. For example, if the first two bits of an IP address are 1-0, the dividing point falls between the 15th and 16th bits. This simplified the routing system during the early years of the Internet because the original routing protocols did not supply a “deciphering key” or “mask” with each route to identify the length of the network-prefix.

Class A Networks (/8 Prefixes)
Each Class A network address has an 8-bit network-prefix with the highest order bit set to 0 and a seven-bit network number, followed by a 24-bit host-number. Today, it is no longer considered ‘modern’ to refer to a Class A network. Class A networks are now referred to as “/8s” (pronounced “slash eight” or just “eights”) since they have an 8-bit network-prefix.

A maximum of 126 (2 7 -2) /8 networks can be defined. The calculation requires that the 2 is subtracted because the /8 network 0.0.0.0 is reserved for use as the default route and the /8 network 127.0.0.0 (also written 127/8 or 127.0.0.0/8) has been reserved for the “loopback” function. Each /8 supports a maximum of 16,777,214 (2 24 -2) hosts per network. The host calculation requires that 2 is subtracted because the all-0s (“this network”) and all-1s (“broadcast”) host-numbers may not be assigned to individual hosts.

Since the /8 address block contains 2 31 (2,147,483,648 ) individual addresses and the IPv4 address space contains a maximum of 2 32 (4,294,967,296) addresses, the /8 address space is 50% of the total IPv4 unicast address space.

Class B Networks (/16 Prefixes)
Each Class B network address has a 16-bit network-prefix with the two highest order bits set to 1-0 and a 14-bit network number, followed by a 16-bit host-number. Class B networks are now referred to as”/16s” since they have a 16-bit network-prefix.

A maximum of 16,384 (2 14 ) /16 networks can be defined with up to 65,534 (2 16 -2) hosts per network. Since the entire /16 address block contains 2 30 (1,073,741,824) addresses, it represents 25% of the total IPv4 unicast address space.

Class C Networks (/24 Prefixes)
Each Class C network address has a 24-bit network-prefix with the three highest order bits set to 1-1-0 and a 21-bit network number, followed by an 8-bit host-number. Class C networks are now referred to as “/24s” since they have a 24-bit network-prefix.

A maximum of 2,097,152 (2 21 ) /24 networks can be defined with up to 254 (2 8 -2) hosts per network. Since the entire /24 address block contains 2 29 (536,870,912) addresses, it represents 12.5% (or 1/8th) of the total IPv4 unicast address space.

Other Classes
In addition to the three most popular classes, there are two additional classes. Class D addresses have their leading four-bits set to 1-1-1-0 and are used to support IP Multicasting. Class E addresses have their leading four-bits set to 1-1-1-1 and are reserved for experimental use.

Dotted-Decimal Notation
To make Internet addresses easier for human users to read and write, IP addresses are often expressed as four decimal numbers, each separated by a dot. This format is called “dotted-decimal notation.”

Dotted-decimal notation divides the 32-bit Internet address into four 8-bit (byte) fields and specifies the value of each field independently as a decimal number with the fields separated by dots. Figure 5 shows how a typical /16 (Class B) Internet address can be expressed in dotted decimal notation.

Figure 5: Dotted-Decimal Notation

Table 1 displays the range of dotted-decimal values that can be assigned to each of the three principle address classes. The “xxx” represents the host-number field of the address which is assigned by the local network administrator.

Table 1: Dotted-Decimal Ranges for Each Address Class

Unforeseen Limitations to Classful Addressing
The original designers never envisioned that the Internet would grow into what it has become today. Many of the problems that the Internet is facing today can be traced back to the early decisions that were made during its formative years.

• During the early days of the Internet, the seemingly unlimited address space allowed IP addresses to be allocated to an organization based on its request rather than its actual need. As a result, addresses were freely assigned to those who asked for them without concerns about the eventual depletion of the IP address space.
• The decision to standardize on a 32-bit address space meant that there were only 2 32
  (4,294,967,296) IPv4 addresses available. A decision to support a slightly larger address space would have exponentially increased the number of addresses thus eliminating the current address shortage problem.
• The classful A, B, and C octet boundaries were easy to understand and implement, but they did not foster the efficient allocation of a finite address space. Problems resulted from the lack of a network class that was designed to support medium-sized organizations. A /24, which supports 254 hosts, is too small while a /16, which supports 65,534 hosts, is too large. In the past, the Internet has assigned sites with several hundred hosts a single /16 address instead of a couple of /24s addresses. Unfortunately, this has resulted in a premature depletion of the /16 network address space. The only readily available addresses for medium-size organizations are /24s which have the potentially negative impact of increasing the size of the global Internet’s routing table.

The subsequent history of Internet addressing is focused on a series of steps that overcome these addressing issues and have supported the growth of the global Internet.

Additional Practice with Classful Addressing
Please turn to Appendix B for practical exercises to further your understanding of Classful IP Addressing.

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