IP address
An IP ("Internet Protocol") address is a unique identification number used by networked electronic devices. Each device has a unique IP address (or is behind a NAT router which does) on the Internet, which is used much like a mailing address. All communications sent to a specific device must list that device's IP address, and all communications from that device must include its source address.
Static vs dynamic
IP addresses cannot be randomly chosen by a device; that device must be assigned an address by the network it is connecting to. This address can be assigned as either static or dynamic. Static IP addresses are permanently assigned to a specific device. This is helpful when external devices need to initiate contact with that device (such as a web server receiving a visitor). Dynamic IP addresses are issued by a DHCP server from a pool of IP addresses for a limited time, when devices request them. When a device's dynamic IP address expires, it is either renewed, or the device is assigned a new one.
IP versions
IP version 4 is currently used by most network devices. However, with increasing numbers of electronic devices accessing the internet, available IPv4 addresses have been essentially depleted. IPv4 uses 32-bit numeric strings which limits it to 4,294,967,296 possible addresses.
IP version 6 is the replacement for the aging IPv4. This protocol theoretically offers 2^128 (340,282,366,920,938,463,463,374,607,431,768,211,456) unique addresses.[1][2] However, RFC 2374 (obsoleted by RFC 3587) has limited the valid IPv6 addresses to about 2^125 at this time. Additionally, some addresses will be reserved for special purposes, and for various other reasons, not all of these theoretical addresses will but usable.[3] With IPv6, the amount of possible IP addresses is increased exponentially, allowing for more exponential growth in the use of mobile phones, literally all of which now have internet connectivity, computers, video game consoles such as the PlayStation 3 and the Xbox 360, tablet computers, and even computers found within modern automobiles. IPv6 can also justify for the future growth in world population of Planet Earth.
Tracking
IP address tracking tools have been used by stalkers and cyber criminals to pinpoint the physical location of their victims house based upon, for example, a Wikipedia® article "edit history" page that reveals the IP address of an editor who did not log in with a registered username. Although in some instances, many IP addresses trace back to one location that contains a network hub owned by an internet service provider. This exact same technique can be used by the webmasters of major websites such as YouTube and Facebook to help law enforcement track down a suspected child molester or a troll who was reported for making threats.
IP address tracking plays a vital role in law enforcement, such as the track down and prosecution of hackers, child molesters, stalkers, and even potential terrorists. internet service providers also play a role in assisting law enforcement, although the downloading of copyrighted songs, movies, games, and programs is still not widely considered illegal by the general public or by law enforcement agencies as long as it is only for personal use.