TCP/IP defines five classes of IP addresses: class A, B, C, D, and E. Each class has a range of valid IP addresses. The value of the first octet determines the class. IP addresses from the first three classes (A, B and C) can be used for host addresses. The other two classes are used for other purposes – class D for multicast and class E for experimental purposes.
The system of IP address classes was developed for the purpose of Internet IP addresses assignment. The classes created were based on the network size. For example, for the small number of networks with a very large number of hosts, the Class A was created. The Class C was created for numerous networks with small number of hosts.
For the IP addresses from Class A, the first 8 bits (the first decimal number) represent the network part, while the remaining 24 bits represent the host part. For Class B, the first 16 bits (the first two numbers) represent the network part, while the remaining 16 bits represent the host part. For Class C, the first 24 bits represent the network part, while the remaining 8 bits represent the host part.
Consider the following IP addresses:
10.50.120.7 – because this is a Class A address, the first number (10) represents the network part, while the remainder of the address represents the host part (50.120.7). This means that, in order for devices to be on the same network, the first number of their IP addresses has to be the same for both devices. In this case, a device with the IP address of 10.47.8.4 is on the same network as the device with the IP address listed above. The device with the IP address 11.5.4.3 is not on the same network, because the first number of its IP address is different.
172.16.55.13 – because this is a Class B address, the first two numbers (172.16) represent the network part, while the remainder of the address represents the host part (55.13). A device with the IP address of 172.16.254.3 is on the same network, while a device with the IP address of 172.55.54.74 isn’t.
Special IP address ranges that are used for special purposes are:
- 0.0.0.0/8 – addresses used to communicate with the local network
- 127.0.0.0/8 – loopback addresses
- 169.254.0.0/16 – link-local addresses (APIPA)
Private IP Addresses
The original design of the Internet intended that each host on every network should have a real, routable IP address. An organization that would like to access the Internet would complete some paperwork to describe its internal network and the number of hosts on it. The organization would then receive a number of IP addresses, according to its needs. But there was one huge problem with this concept – if each host on each network in the world was provided with an unique IP address, we would have run out of IP addresses a long time ago!
Therefore, the concept of private IP addressing was developed to address the IP address exhaustion problem. The private IP addresses can be used on the private network of any organization in the world and are not globally unique.
Consider the following example:
In the example above you can see that two unrelated organizations use the same private IP network (10.0.0.0/24) inside their respective internal networks. Because private IP addresses are not globally unique, both organizations can use private IP addresses from the same range. To access the Internet, the organizations can use a technology called Network Address Translation (NAT), which we will describe in the later lessons.
There are three ranges of addresses that can be used in a private network (e.g. your home LAN or office)
- 10.0.0.0 – 10.255.255.255
- 172.16.0.0 – 172.31.255.255
- 192.168.0.0 – 192.168.255.255
Internet routers are configured to discard any packets coming from the private IP address ranges, so these addresses are not routable on the Internet.