Network performance monitoring in a content delivery system

A method for Internet content delivery in a content delivery network established at network locations, the content delivery network comprising a plurality of content servers for serving content resources. The plurality of content servers includes a plurality of subsets of content servers, each subject being located at one of a plurality of Internet data centers. For each Internet Protocol (IP) address block from which requests for content resources are expected to be received, the method generates a candidate list of Internet data centers to be used to service the requests for content resources. For the IP address block, the method selects at least one of the Internet data centers from the candidate list to be used to service the requests for content resources. The selected Internet data center for the IP address block is written into a network map. The selecting step is carried out concurrently for each IP address block from which requests for content resources are expected to be received such that the network map comprises the selected Internet data center for each IP address block. The network map is then provided to a domain name service (DNS) associated with the content delivery network. In response to a DNS query received at the domain name service associated with the content delivery network, the network map is used to identify one of the Internet data centers from the candidate list to be used to service a request for a content resource.