Content delivery networks (CDNs) are systems that span wide geographical areas and work to optimize the delivery of content. Because of their server placement within and across the internet, CDNs utilize unicast for global distribution. Their servers replicate and distribute content, for both live video and VOD, to the best distribution point for the viewer. Their servers are dynamic and change with load and distribution patterns. This approach is not ideal on a cost basis, when users are known to be using the same network. However, despite potential cost implications, the main advantage to CDN is that it does enable hugely distributed audiences to be served, regardless of location.