IP v4, IP v6
IP version 4 (IPv4) is the main version of the Internet Protocol. This version is currently used by almost all IP networks. An IP version 4 address is a 32-bit number that is typically written as four decimal numbers separated by periods. An example is "192.168.1.3".
As a convention, an IP address that ends in ".1" usually refers to a gateway or router on a particular network. An address that ends in ".255" is a so-called broadcast address: all devices in the same network should handle packets addressed to the broadcast address.
IP version 6 (IPv6) addresses were introduced because the old IP version 4 addresses were in danger of running out. An IP version 6 address is a 128-bit number that is typically written as eight groups of four hexadecimal digits. The groups are separated by colons. An example is "2001:610:113b:50a1::136".
Originally, when the Internet Protocol was first defined, IP (version 4) addresses were handed out to organizations in blocks. There are three classes of blocks: Class A, Class B and Class C. The higher the class, the larger the number of IP addresses in the block.
The class an IP address belongs to follows from the first decimal number: Class A addresses have numbers between 1 and 127, Class B is between 128 and 191, and Class C is 192 and higher. There are also Class D (224 to 247) and Class E (248-255), but these are rarely used in practice.
The organization is itself responsible for dividing the IP addresses in its assigned block to nodes in its IP network. For example, the Eindhoven University of Technology has been assigned the Class B block of 131.155, and so can use any IP address between 131.155.0.1 and 131.155.255.254. The ".0.0" and ".255.255" addresses are reserved.
It's clear that this method of dividing up the IP address blocks quickly runs out of addresses. A Class B and especially a Class A block gives an enormous amount of addresses to one organization, which probably does not need all of them. However the remaining unneeded parts of the block cannot be reassigned to someone else.
To allow a more fine-grained way of handing out addresses, today most IP version 4 address blocks are handed out as subnets. This approach avoids the class-based division and its coarse-grained distribution of IP addresses. For IPv6, classless assignment is the only way to obtain blocks of IP version 6 addresses.
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