IP Addresses and What They Mean

Your IP address is to the Internet (or to the other computers on your local network) much the same as your street address is to your mail carrier. It uniquely identifies your computer using a simple, 32-bit addressing scheme. This scheme, which originated in the late 1960s and early 1970s, uses four octets separated by dots, in the form w.x.y.z (we'll refer to these letters representing each octet throughout the chapter), to describe both the network's address and the local machine's address on that network. Each octet is represented by a single decimal number but is called an octet because it requires 8 bits to describe.

In terms of IP addresses, all networks fall into one of three classes: A, B, or C. These different classes describe networks (sometimes referred to as licenses) of very different sizes and complexities. The licenses to use a range of IP addresses are controlled by Network Solutions, successor to the Internet Network Information Center (InterNIC).

Class A Networks

A class A network has an address that begins with a number from 1 through 127 for the first octet—the w portion of the address. This octet describes the network itself, and the remainder of the address is the actual local device's address on that network. A class A network with the network address of 10 (the w portion) contains all IP addresses from 10.0.0.0 to 10.255.255.255.

The class A address 127 has a special meaning and isn't available for use. This means there are a total of 126 possible usable class A addresses in the world (from 1 through 126), and that each class A network can contain more than 16 million unique network devices.

The class A addresses were spoken for long ago and are assigned to such entities as the Department of Defense, Stanford University, and Hewlett-Packard.

Real World

127: The Loopback Address

All IP addresses that begin with the network number 127 are special. Your network card interprets them as loopback addresses. Any packet sent to an address beginning with 127 is treated as if it had gotten to its intended address, and that address is the local device. So packets addressed to 127.0.0.1 are treated the same as packets to 127.37.90.17; both are actually addressed to your current machine, as are all the other 16 million addresses in the 127 class A network. (You too can have your very own class A network. Of course, you can talk only to yourself, but so what?)

Class B Networks

A class B network uses the first two octets, w and x, to describe the network itself, and the remainder of the address is the actual local device's address on that network. The first octet in a class B network must begin with a number from 128 through 191, resulting in approximately 16,000 class B networks, each of which can have approximately 64,000 unique addresses. This is still a pretty large network, and most of the class B networks were assigned long ago to large organizations or companies such as Rutgers University and Toyota Motor Corporation.

Many of the addresses in the class B address space have been broken up into smaller groups of addresses and reassigned. Large Internet service providers (ISPs), for example, use this technique to more efficiently use the available address space.

Class C Networks

A class C network has an address that begins with a number from 192 through 223 for the w octet of the address and uses the first three octets (w.x.y) to describe the network itself. The last octet, z, describes the actual local device's address on that network. This arrangement makes for roughly 2 million class C networks, each of which can have a maximum of 254 devices on the network. That's enough for a small business or a department, but not for a major corporation.

Class D and Class E Addresses

An IP address with a number from 224 through 239 for the w octet of the address is known as a class D address, which is used for multicast addresses. With multicast addresses, a number of computers can share a single multicast address, in addition to their normal IP addresses. This makes it easy to send identical data to multiple hosts simultaneously—just send the data to the shared multicast address and every member of the multicast group receives it.

The IP address space that uses the numbers from 240 through 247 for the w octet is referred to as a class E address. This space is reserved for future use.

Real World

IP Addresses for Networks that Use Firewalls or Internet Gateways

Suppose you know that your internal, private network will never be directly connected to the Internet. This is becoming increasingly common, as companies are moving toward sheltering their entire network behind proxy servers and firewalls. Should you use any old numbers then? No, you really shouldn't. There is a special set of network addresses reserved for just such uses. These addresses

are defined in RFC 1918, and by using these addresses you can comfortably employ a substantially larger address space than you would otherwise be able to access.

Using these special network addresses also protects the integrity of the Internet. There have already been instances in which networks using addresses that were already assigned to other organizations were connected to the Internet, causing substantial problems. Because these special addresses are officially only for private networks, they are automatically filtered at routers, protecting the Internet. The following is a list of these special addresses:

• 10.0.0.0 through 10.255.255.255 (a class A network)
• 172.16.0.0 through 172.31.255.255 (16 contiguous class B networks)
• 192.168.0.0 through 192.168.255.255 (256 contiguous class C networks)

If you need to create a test network, for example, or have some other reason to be sure you won't ever be connecting to the Internet, but you need to use TCP/IP for your network protocol, choose from this special set of addresses. You should also use these addresses for your internal network when you connect to the Internet only through a proxy server or firewall that shields your actual IP address from being seen. Microsoft Internet Security and Acceleration Server is such a firewall. When you use a firewall or proxy server, you require "real" IP addresses only for machines that are outside your firewall and are visible to the Internet as a whole, saving on IP address space.