A content delivery network (CDN) is a service provided primarily to website owners who publish large amounts of media. The CDN acts as a distributor, hosting clones of content for the website, taking a large amount of the load off of the site’s own server and distributing it over diverse geographical locations. Rather than receiving an image or video directly from the site’s server, for example, a user may receive it from a CDN.
A CDN can range from simple web caching (hosting identical web content) to full remote application delivery. With a web cache, the most frequently accessed information is stored on the CDN, reducing bandwidth, dedicated server load, and client (browser) response-time. A CDN can be used for load-balancing, where server content is duplicated and distributed from diverse locations. With application delivery, the CDN acts as an application service provider (ASP) providing software as a service (SaaS), which is what most people envision when they speak of cloud computing.
Peer-to-peer (P2P) CDNs are becoming more popular, with major corporations like AT&T making use of the technology to send large amounts of data without using as much centralized bandwidth and server load. P2P more evenly distributes the load, making it easier to deliver content like streaming HD video.
Popular commercial CDNs include Akamai, Amazon CloudFront, Internap, Limelight Networks, CacheFly, and Windows Azure. Some free CDNs are Coral Content Distribution Network, FreeCast, PPLive, QQLive, and PPStream. Among the commercial CDNs that use P2P technology are BitTorrent, Pando, and AT&T.