Although the principle of peer to peer (P2P) has been around since the days of Gopher and FTP, it really took off in the late 90s. The arrival of MP3 - a file compression standard that allowed near CD-quality music to be squeezed to around 1MB per minute - made music compact enough to transfer over the internet.
The first killer application was Napster, which allowed people to search for and swap MP3 files over the net. It attracted music lovers, pirates and, eventually, lawyers, furious that copyright material could be swapped openly. When Napster finally buckled under legal pressure last year, it was replaced by a flood of pretenders.
There are at least a dozen P2P clients, mainly linked to one of two standards - Gnutella which includes Limewire, Bearshare and Morpheus, and FastTrak, with the market leader, KaZaA. It claims more than 250m users worldwide.
By downloading and installing a P2P client, your hard drive becomes part of a global, file-sharing community, with all the legal and technical consequences it entails. So an important first step is to designate which types of file you wish to share, which areas of your system you want open to others, and how much bandwidth you wish to allocate for uploads and downloads. Then you're ready to go.
The main reason P2P has been so popular is not that it's criminal, but that it's criminally easy to use. Simply enter the title of the track or the artist's name and the screen fills with other file sharers who appear to have what you seek. You can see the sound quality of the track (128kbps is close enough to CD) and the bandwidth available for download (ADSL is enabling ever-faster transfer speeds and fewer lost connections). Many P2P clients also include a basic media player, allowing you to preview the file as it downloads - increasingly important as the record industry floods P2P networks with fake files.
Three important things have changed since the days of Napster. First, user information is no longer stored on a central server, making KaZaA et al less vulnerable to litigation. Second, P2P is now as much about swapping video or complete software applications as it is about music - providing a new headache for copyright owners. Finally, P2P has become a magnet for hackers and malcontents, with fake files, Trojan horses and pop-ups slowly infecting all the networks. KaZaA is even stuffed with its own spyware cookies, although some can be avoided by downloading the ad-free Kazaa Lite instead.
This increasing lack of trust between file sharers is one reason subscription-based services are gathering momentum. Last week, Apple's iTunes notched up its one millionth download and Napster is in the process of re-launching in a similar guise, with fully licensed tracks costing as little as 99 cents each. After years of losing a bitter struggle against online piracy, perhaps sensible pricing was the answer all along.
