IP addresses are usually not entered directly by end users. Instead, DNS servers are used to map permanent and user-friendly names like boutell.com to unfriendly and impermanent IP addresses, such as 220.127.116.11.
An IP address is made up of four numbers, each between 0 and 255. For instance, as of this writing, the IP address of boutell.com is:
The most general information is conveyed by the first number, and the specific identification of a single computer within a single network is usually made by the last number. In general, delegation of responsibility for various portions of the IP address space is carried out by the Asia Pacific Network Information Centre (APNIC), the American Registry for Internet Numbers (ARIN), the Latin-American And Caribbean Internet Addresses Registry (LACNIC), and the RIPE Network Coordination Centre (RIPE NCC).
The above description applies to IPv4, the most commonly used version of the IP protocol that underlies the Internet and similar networks. A newer system, IPV6, addresses the fact that the number of IPv4 addresses is limited to approximately four billion (256 to the fourth power), with the practical maximum considerably lower than that due to the ways in which addresses are assigned. When much of the Earth's population begins to use the Internet from a variety of devices, this limitation becomes a serious problem. IPv6 addresses have a vastly greater range, inexhaustible for all practical purposes.
You will not always have the same IP address, unless you have specifically arranged for a fixed IP address, typically from a cable modem, DSL or other high-speed provider. Therefore, your IP address usually does not uniquely identify you as an individual. When you dial into your Internet service provider with your modem, an IP address is temporarily assigned to your computer for the duration of the call. Even web servers such as boutell.com will typically change their IP address when they move from one hosting facility to another; DNS servers make this transparent for the end user by automatically translating domain names to IP addresses. With the exception of the "root" DNS servers, which are used to resolve the IP addresses of all other DNS servers, all IP addresses are subject to potential change.
Those who use the Internet at work, or who have a connection-sharing router at home, do not truly have an Internet IP address for their individual computer. Instead, the connection-sharing router holds the Internet IP address, carries out the requests made by the various personal computers "behind" the router, and appears to the rest of the Internet to be a single, very busy computer. The personal computers "behind" the router have IP addresses on an intranet. Such IP addresses typically resemble 192.168.2.2 or 10.1.1.7, because the prefixes 192.168. and 10. are universally reserved for such private networks and are guaranteed never to be assigned to systems on the Internet.
This lack of a true Internet IP address for each personal computer can be a very good thing, because it prevents incoming connections to individual PCs, providing some protection from certain types of attacks. Unfortunately, there are many other ways for computers to become infected by viruses, spyware and similar software. For more information, see can my computer catch a virus from a web page? and why is my web browser broken?
Such setups can also have a downside: if you wish to run a server on one of the computers behind the connection-sharing router, you must explicitly configure your router to forward connections on certain ports to that particular computer.
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