IP addresses can be split into two sections, the network ID, and the host ID. Earlier, we mentioned that IBM owns all IP addresses that having a nine as the value of the first octet in an IP address. If we take an example IP address of 220.127.116.11, the network ID would be the first octet, and the host ID, would be the second, third and fourth octets. The address class system is a way of defining how the global IP address space is split up. There are three primary types of address classes. Class A, class B, and class C. Class A addresses are those where the first octet is used for the network ID, and the last three are used for the host ID. Class B addresses are where the first two octets are used for the network ID, and the second two, are used for the host ID. Class C addresses, as you might have guessed, are those where the first three octets are used for the network ID, and only the final octet is used for the host ID. Each address class represents a network of vastly different size. For example, since a class A network has a total of 24 bits of host ID space, this comes out to two to the twenty-fourth, or 16,777,216 individual addresses. Compare this with a class C network, which only has eight bits of host ID space. For a class C network, this comes out to two to the eighth, or 256 addresses. You can also tell exactly what address class an IP address belongs to just by looking at it. If the very first bit of an IP address is a zero, it belongs to a class A network, if the first bits are one, zero, it belongs to a class B network. Finally, if the first bits are 110, it belongs to a class C network. Since humans aren't great at thinking in binary, it's good to know that this also translates nicely to how these addresses are represented in dotted decimal notation. You might remember that each octet in an IP address is eight bits, which means each octet can take a value between zero and 255. If the first bit has to be a zero, as it is with the class A address, the possible values for the first octet are zero through 127. This means that any IP address with a first octet with one of those values is a class A address. Similarly, class B addresses are restricted to those that begin with the first octet value of 128 through 191, and class C addresses begin with the first octet value of 192 through to 223. You might notice that this doesn't cover every possible IP address. That's because there are two other IP address classes, but they're not quite as important to understand. Class D addresses always begin with the bits 1110, and are used for multicasting, which is how a single IP datagram can be sent to an entire network at once. These addresses begin with decimal values between 224 and 239. Lastly, class E addresses make up all of the remaining IP addresses, but they're unassigned and only used for testing purposes. In practical terms, this class system has mostly been replaced by a system known as CIDR or Classless inter-domain routing. But the address class system is still in place in many ways, and is important to understand for anyone looking for a well rounded networking education, and you know we're all about that. So don't worry, we'll be covering CIDR in a future lesson.