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: 8 bits for Class A, 16 bits for Class B, and 24 bits for Class C. Subnetting lets you select an arbitrary number of bits to use for the network ID.
Two reasons compel people to use subnetting. The first is to allocate the limited IP address space more efficiently. If the Internet were limited to Class A, B, or C addresses, every network would be allocated 254, 64,000, or 16 million IP addresses for host devices. Although many networks with more than 254 devices exist, few (if any) exist with 64,000, let alone 16 million. Unfortunately, any network with more than 254 devices would need a Class B allocation and probably waste tens of thousands of IP addresses.
The second reason for subnetting is that even if a single organization has thousands of network devices, operating all those devices with the same network ID would slow the network to a crawl. The way TCP/IP works dictates that all the computers with the same network ID must be on the same physical network. The physical network comprises a single broadcast domain, which means that a single network medium must carry all the traffic for the network. For performance reasons, networks are usually segmented into broadcast domains that are smaller than even Class C addresses provide.
Subnets
A subnet is a network that falls within a Class A, B, or C network. Subnets are created by using one or more of the Class A, B, or C host bits to extend the network ID. Thus, instead of the standard 8-, 16-, or 24-bit network ID, subnets can have network IDs of any length.
Figure 3-3 shows an example of a network before and after subnetting has been applied. In the unsubnetted network, the network has been assigned the Class B address 144.28.0.0. All the devices on this network must share the same broadcast domain.
