What is the purpose of the different classes of IP addresses?

Answer 1

The 5 classes of IP addresses were created as the original internet routing scheme in the 1970's.

These 5 classes were proposed by the IEEE committee.

These classes were not created, after the fact, to meet the needs of an ever expanding Internet.

When the IEEE proposed these classes; each of the 5 classes were 'designed' for a specific purpose.

Class A was designed to meet the needs of large networks.

This class will only support 126 networks; but each network can support 16,777,214 hosts.

Class B was designed for medium-sized networks.

This class will support 16,384 networks; and limited to 65,534 hosts per network.

Class C was designed for small networks; thus the number of hosts per network will be small, however it will support many more networks total.

Class C supports 2,097,152 networks; but only 254 hosts per network.

All Class A addresses are already used up and Class B addresses are rather difficult to obtain.

Most new connections are assigned Class C addresses.

However; if Class C does not meet the requirements of an organization, multiple and multiple Class C addresses can be assigned to fill the need for Internet connectivity (because has mentioned Class C only supports 254 hosts per network, so if their network is large they will have to use many of them).

To understand these classes, just know that they were designed with simple arithmetic.

The number of networks allowed per Class is just the number of possible Network ID's for a given Class range.

The number of hosts allowed per Class is just the number of possible Host ID's for a given Class range.

For example Class A has range of 1.0.0.0-126.0.0.0 & uses only the first octet (1.-126.)

to define the Network ID portion of each IP address in the range of Class A. As you remember Class A supports only 126 different networks. Thus, you can see the simple arithmetic involved; the number of supported networks is a function of the number of possible Network ID's in the range of A.

However; as stated earlier Class A was designed for LARGE networks. To accomplish this Class A only uses the first octet to define the Network ID portion of each IP address; so that means it has a full 3 octets to define the Host ID portion.

Which means the total number of possible Host ID's in the range of A is a whopping 16,777,214. To calculate the total number of possible Host ID's in Class A, we must add the total possible permutations of each octet. The allowable range for an octet is 0-255 (I'll assume you know binary & why each octet has a range of 0-255). Since zero is a possibility, the possible permutations of each octet is 256. And we know that Class A uses 3 octets for the number of Host ID's. Thus to calculate the total possible permutations we simply multiply 256 by itself 3 times. Like so, 256x256x256=16,777,216 possible Host ID's for Class A.

But, it is not quite that simple as you can see our calculation is off by 2. The reason for this is that 2 possibilities cannot be assigned to a host.

These are: .0.0.0 & .255.255.255.

The reason for this is that .0.0.0 is not a defined host range for the Host ID portion of an IP address, because this address along with its subnet mask is used to identify the network; for example the 10.0.0.0/8 network. The Network ID + .255.255.255 for the Host ID is a special address used as a Directed Broadcast; for example 10.255.255.255. This special address can be used as a destination, but cannot be assigned to any host.

So this is how the IEEE 'designed' the IP address Classes; I hope now it is a little easier to see how the IEEE created these Classes and why. Basically, all these classes do is designate different sized networks on the Internet. They were created as the original Internet routing scheme and are used by routers.

And as for Class D and E; these address classes are reserved.

Class D addresses are reserved for multicasting &

Class E addresses are reserved for experimental purposes.

For all practical purposes, Classes D & E are invalid for assigning host IP addresses on a typical TCP/IP network.

Remember these Classes were proposed in the 1970's, with modern technology, they have very little meaning in the overall scheme anymore-- As IP version 6 is now being implemented and there are much less limitations with IPv6. IP version 6 allows for many many more addresses to be used than IPv4 as IP version 4 addresses are running out and IPv6 will be the addressing of the future.

IPv4 only allows for 4,294,967,295 (about 4 billion addresses), this is calculated by the fact that an IP address is a 32 bit address ( four octets or 4x8=32). To calculate the possible no. of addresses we take 2^32 ( 2 to the 32nd power) = 4,294,967,296.

IPv6 allows for 340,282,366,920,938,463,463,374,607,431,768,211,456 addresses;

IPv6 uses a 128 bit hexadecimal address. So, we calculate this number by taking

2 to the 128th power (2^128).

So IPv4 Classes hold very little meaning anymore from their original designated purpose.

Especially when you take into account the new technology and the methods being used to implement networks today.

For example many networks use private addressing for their internal networks; which by the way private IP addresses have no meaning outside of their intranet and cannot be used to access the internet. So, many networks use private addressing to shield their internal networks from the internet, this does not mean that clients on the network cannot access the internet however. An IP Gateway is used for external access to outside networks (such as the Internet). How an IP Gateway works is that it uses Network Address Translation (NAT); it translates private IP addresses to a valid IP address that can be used to access the Internet (typically using a range of Class C addresses). This has the effect of hiding the internal network from the Internet.

As an example let's say that a network of 2,000 computers is using private IP addressing; but only has a couple hundred computers which need access to the internet at any one time. So that, you can see valid addresses are only used when a client is actually accessing the internet; this saves a lot of addresses from being horded so no one else can use them.

There are many different ways to set up a network and many different internetworking devices that have the effect of 'saving' IP addresses, so that they won't run out or so that networks can be implemented beyond the limitations of the original IP address Classes. Internetworking devices such as an IP Gateway are implemented more for security than a fear of running out of addresses.

So, I took the time you write this answer, because at one point I inevitably asked this same question and many more are bound to ask it. The answer is that IPv4 Classes really don't have much of a purpose anymore; because the Internet has outgrown their intended purpose.

Which prompts the need for a new addressing scheme-- IPv6.

With IPv6 we're not going to run out of addresses any time soon.