【My Study Note】IP Address Classes
IP Address Classes
IP addresses can be split into two sections, the network ID and the host ID.
IBM owns all IP addresses that have a nine as the value of the first octet in an IP address. If we take an example IP address of 9.100.100.100, 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
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
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
Class C addresses are those where the first three octets are used for the network ID, and only the final octet is used for the host ID.
Difference between classes
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 2 to the 24th 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 2 to the 8th or 256 addresses.
You can tell the address classes
You can also tell exactly what address class in IP address belongs to just by looking at it. If the very first bit of an IP address is 0, it belongs to a Class A network. If the first bits are 10, 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 0 and 255.
If the first bit has to be a 0, as it is with the Class A address, the possible values for the first octet are 0 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 223.
Multicasting
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.
Class E addresses make up all of the remaining IP addresses. But they are unassigned and only used for testing purposes. In practical terms, this class system has mostly been replaced by a system known as CIDR (Classless Inter-Domain Routing).