With the implementation of the Internet, its designers decided to create classes of networks based on the size of the network; each network address belongs to a specific class. The class to which each network belongs initially determines the network ID and the host ID portions of the address, along with the number of hosts compatible with that network.
These groups or blocks are defined in RFC 1700 in what was called classful addressing, where the blocks were defined permanently, i.e., smaller blocks of each of them could not be used. The following is an explanation of each of the defined blocks classes.
Class A Addresses
A class A address has a default subnet mask of 255.0.0.0.0, which means that the first octet is the network ID and the last three octets belong to the host ID part of the address. Each octet can contain 256 possible values (0-255), so a class A address supports 16,777,216 hosts on the network (256 × 256 × 256 × 256). In reality, there are only 16,777,214 valid addresses for systems, as there are two addresses on each IP network that are not allowed to be assigned to systems because they are reserved. These are addresses with all host bits set to 0’s (the network ID) and all host bits set to 1’s (the broadcast address). So with a class A address, you will not be able to assign x.0.0.0.0 or x.255.255.255 (where x is your network ID) of all the hosts on the network.
You can always identify a class A address because the value of the first octet is between the numbers 1 and 126. An address that begins with 127 is also a class A address, but you are not allowed to use any address that begins with 127 because it is reserved for loopback addresses. For example, IP address 8.2.2.10 is a class A address because the first octet is 8, and it is in the range 1 to 126.
Class B addresses
Class B addresses have a default subnet mask of 255.255.0.0.0, which means that the first two octets are the network ID and the last two octets are the host part of the address. This means that we can have 65,536 hosts (256 × 256) on the network, but remember that the first and last addresses are the same as in class A.
Because of the number of hosts compatible with a Class B address, you will typically find that a medium-sized company has a class B address. You can identify a class B address because the first octet begins with a number between 128 and 191.
Direcciones Clase C
Class C addresses have a subnet mask of 255.255.255.0, which means that the first three octets are the network ID and the last octet is the host ID. Having only one octet as the host ID means that a class C address can support only 254 hosts (256 – 2) on the network.
You can identify a Class C address because it has a value for the first octet from 192 to 223. For example, an IP address of 202.45.8.6 is a Class C address because 202 is between 192 and 223. It also knows that this system has a subnet mask of 255.255.255.0 because it is a class C address.
Class D addresses
Class D addresses are used for special types of applications on the network known as multicast applications. Multicast applications send data to multiple systems at the same time, sending data to the multicast address, and anyone who has registered with that address will receive the data. A class D multicast address will not be specifically assigned to hosts on the network for normal communication.
Class D addresses have a value in the first octet ranging from 224 to 239. With that many ranges, Class D has the potential for 268,435,456 unique multicast groups that the user can subscribe to from a multicast application.
Class E Addresses
Class E addresses were designed and reserved for experimental purposes only, so you will never see a Class E address on a network. Class E addresses have the first octet with a value in the range 240-247.
Conclusion
In summary, look at the following tables:
| Class | First octet range | Network subnet | Initial IP | End IP | Networks | Host |
| A | 0 – 127 | 255.0.0.0 | 0.0.0.0 | 127.255.255.255 | 128 (27) | 16777214 (224-2) |
| B | 128-191 | 255.255.0.0 | 128.0.0.0 | 191.255.255.255 | 16384 (214) | 65534 (216-2) |
| C | 192-223 | 255.255.255.0 | 192.0.0.0 | 223.255.255.255 | 2097152 (221) | 254 (28-2) |
| D | 224-239 | 255.255.255.255 | 224.0.0.0 | 239.255.255.255 | Reserved for Multicasting | |
| E | 240-254 | 255.255.255.255 | 240.0.0.0 | 254.255.255.255 | Experimental |
Note: Concerning Class A, the address 0.0.0.0.0 is used to represent a default route, that is, a route that includes all the others. We will clarify this point later. Another detail to take into account is that the segment that starts with 127.0.0.0.0 is reserved for loopback and diagnostic addresses.
