number is so large it defies understanding. If the Internet Assigned Numbers Authority (IANA) had been around at the creation of the universe and started handing out IPv6 addresses at a rate of one per millisecond — that is, 1,000 addresses every second — it would now, 15 billion years later, have not yet allocated even 1 percent of the available addresses.
The transition from IPv4 to IPv6 has been slow. IPv6 is available on all new computers and has been supported on Windows since Windows XP Service Pack 1 (released in 2002). However, most ISPs still base their service on IPv4. Thus, the Internet will continue to be driven by IPv4 for at least a few more years.
Note: This is now the eighth edition of this book. Every previous edition of this book, all the way back to the very first edition published in 2004, has had this very sidebar. In 2004, I said that the Internet would continue to be driven by IPv4 for at least a few more years. “A few more years” has morphed into 14 years, and we’re still living in the world of IPv4. Make no mistake: The world will eventually run out of IPv4 addresses, and we’ll have to migrate to IPv6 … in a few years, whatever that means.
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–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, as follows:
Class A: The first bit is zero.
Class B: The first bit is one, and the second bit is zero.
Class C: The first two bits are both one, and the third bit is zero.
Class D: The first three bits are all one, and the fourth bit is zero.
Class E: The first four bits are all one.
Because Class D and E addresses are reserved for special purposes, I focus the rest of the discussion here on Class A, B, and C addresses. Table 3-3 summarizes the details of each address class.
TABLE 3-3 IP Address Classes
| Class | Address Number Range | Starting Bits | Length of Network ID | Number of Networks | 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.
Only about 40 Class A addresses are actually assigned to companies or organizations. The rest are either reserved for use by the Internet Assigned Numbers Authority (IANA) or are assigned to organizations that manage IP assignments for geographic regions such as Europe, Asia, and Latin America.
In case you’re interested, you can find a complete list of all the Class A address assignments at www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xml.
You may have noticed in Table 3-3 that Class A addresses end with 126.x.y.z, and Class B addresses begin with 128.x.y.z. What happened to 127.x.y.z? This special range of addresses is reserved for loop-back testing, so these addresses aren't assigned to public networks.
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, in order to indicate that the address is a Class B address. As a result, 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.
The problem with Class B networks is that even though they are much smaller than Class A networks, they still allocate far too many host IDs. Very few networks have tens of thousands of hosts. Thus, 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.
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 is what led to the development of subnetting, which I describe in the next section.
Subnetting
Subnetting is a technique that lets network administrators use the 32 bits available in an IP address more efficiently by creating networks that aren’t limited to the scales provided by Class A, B, and C IP addresses. With subnetting, you can create networks with more realistic host limits.
Subnetting provides a more flexible way to designate which portion of an IP address represents the network ID and which portion represents the host ID. With standard IP address classes, only three possible network ID sizes exist: