IP address

What is IP address?

When you type an email address how does the email system know where to send the mail? When you are on the Web and type a URL, how does the Internet know where to get the information? The answer to both questions, and in fact the answer to any routing question on the Internet or a network, is using Internet Protocol addresses. Each destination on the Internet (whether a server, a mail client, or a machine) has a unique address. So, when data is sent over the Internet, the IP address can be used to determine which servers to send the data to, and where the ultimate destination is.

Currently, the Internet and most networks use the Internet Protocol version 4 (called IPv4) to tell data that is being passed around a network or the Internet where to go. Each destination has this IP address registered with a database to make lookups easier (this is a bit of a simplification, but a useful one). There is a newer version of IP, called IPv6, but it is not been widely used yet.

IPv4 uses a 32-bit address to identify a machine on a network and the network to which it is attached. IP addresses identify a machine's connection to the network, not the machine itself - an important distinction. There are four formats for the IP address, with each used depending on the size of the network. The four formats are called Class A through Class D. Class A addresses are for large networks that have many machines. Class B addresses are for intermediate sized networks. Class C network have only 8 bits for the local or host address, limiting the number of devices to 256. Finally, Class D networks are used for multicasting purposes, when a general broadcast to more than one device is required.

IP addresses are four sets of eight bits, for the total 32 bits. The IP addresses are usually written out in their decimal equivalent, instead of the long binary strings. This is the familiar host address number network users are used to seeing, such as 147.10.13.28. The decimal notation used for IP addresses is properly called "dotted quad notation. It is possible for a device to have more than one IP address if it is connected to more than one network, as is the case with gateways. These devices are called multihomed, as they have a unique address for each network they are connected to.

To make these IP addresses easier for users, they can be translated to common names and letters using the Domain Name System. This can pose a problem, though, as there must be some method of unambiguously relating the physical address, the network address, and a language-based name (such a "brutus" or "bobs_machine").

From the IP address, a network can determine if data is to be sent out through a gateway. Obviously, if the network address is the same as the current address (routing to a local network device, called a direct host), the gateway is avoided, but all other network addresses are routed to a gateway to leave the local network (indirect host). The gateway receiving data to be transmitted to another network must then determine the routing from the data's IP address and an internal table that provides routing information.

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