Classifying IP Addresses | Networking For Dummies

When the original designers of the IP protocol created the IP addressing scheme, they could have assigned an arbitrary number of IP address bits for the network ID. The remaining bits would then be used for the host ID. For example, the designers may have decided that half of the address (16 bits) would be used for the network and the remaining 16 bits would be used for the host ID. The result of that scheme would be that the Internet could have a total of 65,536 networks and each of those networks could have 65,536 hosts.

In the early days of the Internet, this scheme probably seemed like several orders of magnitude more than would ever be needed. However, the IP designers realized from the start that few networks would actually have tens of thousands of hosts. Suppose that a network of 1,000 computers joins the Internet and is assigned one of these hypothetical network IDs. Because that network uses only 1,000 of its 65,536 host addresses, more than 64,000 IP addresses would be wasted.

As a solution to this problem, the idea of IP address classes was introduced. The IP protocol defines five different address classes: A, B, C, D, and E. Each of the first three classes, A through C, uses a different size for the network ID and host ID portion of the address. Class D is for a special type of address called a multicast address. Class E is an experimental address class that isn't used.

The first four bits of the IP address are used to determine into which class a particular address fits:

If the first bit is a zero, the address is a Class A address.
If the first bit is one and the second bit is zero, the address is a Class B address.
If the first two bits are both one and the third bit is zero, the address is a Class C address.
If the first three bits are all one and the fourth bit is zero, the address is a Class D address.
If the first four bits are all one, the address is a Class E address.

Because Class D and E addresses are reserved for special purposes, I focus the rest of this discussion on Class A, B, and C addresses. Table 6-1 summarizes the details of each address class.

Table 6-1: IP Address Classes
Open table as spreadsheet
Class	Address Range	Starting Bits	Length of Network ID	Number of Networks	Number of Hosts
A	1-126.x.y.z	0	8	126	16,777,214
B	128-191.x.y.z	10	16	16,384	65,534
C	192-223.x.y.z	110	24	2,097,152	254

Class A addresses

Class A addresses are designed for very large networks. In a Class A address, the first octet of the address is the network ID, and the remaining three octets are the host ID. Because only eight bits are allocated to the network ID, and the first of these bits is used to indicate that the address is a Class A address, only 126 Class A networks can exist in the entire Internet. However, each Class A network can accommodate more than 16 million hosts.

TECHNICAL STUFF

Only about 40 Class A addresses are assigned to companies or organizations. The rest are either reserved for use by the IANA (Internet Assigned Numbers Authority) or are assigned to organizations that manage IP assignments for geographic regions, such as Europe, Asia, and Latin America.

Just for fun, Table 6-2 lists some of the better-known Class A networks. You probably recognize many of them. In case you're interested, you can find a complete list of all the Class A address assignments at http://www.iana.org/assignments/ipv4-address-space.

Table 6-2: Some Well-Known Class A Networks
Open table as spreadsheet
Net	Description
3	General Electric Company
6	Army Information Systems Center
9	IBM
11	DoD Intel Information Systems
12	AT&T Bell Laboratories
13	Xerox Corporation
15	Hewlett-Packard Company
16	Digital Equipment Corporation
17	Apple Computer, Inc.
18	MIT
19	Ford Motor Company
20	Computer Sciences Corporation
22, 26, 29, 30	Defense Information Systems Agency
34	Halliburton
38	Performance Systems International
40	Eli Lilly and Company
43	Japan Inet
45	Interop Show Network
47	Bell-Northern Research
48	Prudential Securities Inc.
54	Merck and Co., Inc.
56	U.S. Postal Service

Class B addresses

In a Class B address, the first two octets of the IP address are used as the network ID, and the second two octets are used as the host ID. Thus, a Class B address comes close to my hypothetical scheme of splitting the address down the middle, using half for the network ID and half for the host ID. It isn't identical to this scheme, however, because the first two bits of the first octet are required to be 10, to indicate that the address is a Class B address. Thus, a total of 16,384 Class B networks can exist. All Class B addresses fall within the range 128.x.y.z to 191.x.y.z. Each Class B address can accommodate more than 65,000 hosts.

TECHNICAL STUFF

The problem with Class B networks is that even though they're much smaller than Class A networks, they still allocate far too many host IDs. Very few networks have tens of thousands of hosts. Thus, the careless assignment of Class B addresses can lead to a large percentage of the available host addresses being wasted on organizations that don't need them.

Class C addresses

In a Class C address, the first three octets are used for the network ID, and the fourth octet is used for the host ID. With only eight bits for the host ID, each Class C network can accommodate only 254 hosts. However, with 24 network ID bits, Class C addresses allow for more than 2 million networks.

What about IPv6?

Most of the current Internet is based on version 4 of the Internet Protocol, also known as IPv4. IPv4 has served the Internet well for more than 20 years. However, the growth of the Internet has put a lot of pressure on IPv4's limited 32-bit address space. This chapter describes how IPv4 has evolved to make the best possible use of 32-bit addresses, but eventually all the addresses will be assigned-the IPv4 address space will be filled to capacity. When that happens, the Internet will have to migrate to the next version of IP, known as IPv6.

IPv6 is also called IP next generation, or IPng, in honor of the favorite television show of most Internet gurus, Star Trek: The Next Generation.

IPv6 offers several advantages over IPv4, but the most important is that it uses 128 bits for Internet addresses rather than 32 bits. The number of host addresses possible with 128 bits is a number so large that it would make Carl Sagan proud. It doesn't just double or triple the number of available addresses. Just for the fun of it, here's the number of unique Internet addresses provided by IPv6:

340,282,366,920,938,463,463,374,607,431,768, 211,456

This number is so large that it defies understanding. If the IANA had been around at the creation of the universe and started handing out IPv6 addresses at a rate of one per millisecond, it would now, 15 billion years later, have not yet allocated even 1 percent of the available addresses.

Unfortunately, the transition from IPv4 to IPv6 has been a slow one. Thus, the Internet will continue to be driven by IPv4 for at least a few more years.

TECHNICAL STUFF

The problem with Class C networks is that they're too small. Although few organizations need the tens of thousands of host addresses provided by a Class B address, many organizations need more than a few hundred. The large discrepancy between Class B networks and Class C networks led to the development of subnetting, which is described in the next section.