Internet Protocol Tutorial Classes of IP Addresses, IP Broadcast and IP Multicast

IPv4 Address Classes
The IPv4 address space can be subdivided into 5 classes - Class A, B, C, D and E. Each class consists of a contiguous subset of the overall IPv4 address range.

With a few special exceptions explained further below, the values of the leftmost four bits of an IPv4 address determine its class as follows:


All Class C addresses, for example, have the leftmost three bits set to '110', but each of the remaining 29 bits may be set to either '0' or '1' independently (as represented by an x in these bit positions):
110xxxxx xxxxxxxx xxxxxxxx xxxxxxxx
Converting the above to dotted decimal notation, it follows that all Class C addresses fall in the range from 192.0.0.0 through 223.255.255.255.
IP Address Class E and Limited Broadcast
The IPv4 networking standard defines Class E addresses as reserved, meaning that they should not be used on IP networks. Some research organizations use Class E addresses for experimental purposes. However, nodes that try to use these addresses on the Internet will be unable to communicate properly.
A special type of IP address is the limited broadcast address 255.255.255.255. A broadcast involves delivering a message from one sender to many recipients. Senders direct an IP broadcast to 255.255.255.255 to indicate all other nodes on the local network (LAN) should pick up that message. This broadcast is 'limited' in that it does not reach every node on the Internet, only nodes on the LAN.
Technically, IP reserves the entire range of addresses from 255.0.0.0 through 255.255.255.255 for broadcast, and this range should not be considered part of the normal Class E range.
IP Address Class D and Multicast
The IPv4 networking standard defines Class D addresses as reserved for multicast. Multicast is a mechanism for defining groups of nodes and sending IP messages to that group rather than to every node on the LAN (broadcast) or just one other node (unicast).
Multicast is mainly used on research networks. As with Class E, Class D addresses should not be used by ordinary nodes on the Internet.
IP Address Class A, Class B, and Class C
Class A, Class B, and Class C are the three classes of addresses used on IP networks in common practice, with three exceptions as explained next.

source :http://compnetworking.about.com