IP Addresses Explained

Internet Protocol, the IP in TCP/IP and the foundation of Internet communication, requires a source and destination address to deliver information across the Internet. IP addresses, as the name implies, are these addresses, and they uniquely identify every machine on the Internet. It is helpful to think of an IP address as a postal address. If a building does not have a postal address, it cannot receive mail. Likewise, a machine without an IP address cannot receive IP data.

IP addresses are 32 bits long, with every 8 bits forming an information "quad." With 8 bits, each quad can only contain numbers between 0 and 255. A dot (".") between each quad makes it easy, visually, to distinguish quads. Some people describe IP addresses as "dotted quads." You can use this name as a mnemonic to remember what an IP address looks like.
The SlideShow below explains the parts of an IP address.

Every IP address has two logical parts: the network part and the local part. The Network Information Center (NIC) assigns the network part to individual networks, and administrators within each network assign the local part. In both cases, the assigned number must be unique on the represented network.

Different sized networks have different requirements for IP addresses. A network with many hosts uses most of the 32-bit IP address for the local part, while a network with a few hosts uses only some of the bits to identify hosts.
To accommodate differing usage, IP designers broke IP addresses into three address classes: Class A, Class B, and Class C.
Each class divides the quads into network and local parts. More space is allocated for the network part as you move from Class A to Class C. For example, Class A networks use the first quad (8 bits) for the network part, and the remaining three quads (24 bits) for the local part.

Before moving on to the next lesson, click the Quiz link below to check your understanding of IP addresses.
IP Addresses - Quiz