A method of operating a control system (110, 310) to select a content delivery node (120, 130), the method comprising: receiving a domain name lookup request issued by an end user device (140, 340-344) for retrieving content cached by one or more content delivery nodes (120, 130) of a content delivery system; responsive to the domain name lookup request, identifying a network performance profile (111, 311, 312) for the end user device (140, 340-344) based on an internet service provider associated with the end user device (140, 340-344), wherein the network performance profile (111, 311, 312) is created based on network characteristics obtained from at least a second end user device (140, 340-344) that shares the internet service provider with the end user device (140, 340-344), and wherein the network characteristics comprise performance measurements, being made by the at least the second end user device of data transferred between the at least the second end user device (140, 340-344) and at least one of the content delivery nodes (120, 130); selecting a content delivery node (120, 130) of the content delivery system based on the network performance profile; and transferring a network address (121, 131, 615) associated with the selected content delivery node (120, 130) for receipt by the end user device (140, 340-344) responsive to the domain name lookup request; wherein identifying the network performance profile (111, 311, 312) for the end user device (140, 340-344) based on the internet service provider associated with the end user device (140, 340-344) comprises identifying the network performance profile (111, 311, 312) for the end user device (140, 340-344) based on a network address range containing a network address of the end user device (140, 340-344). receiving a domain name lookup request issued by an end user device (140, 340-344) for retrieving content cached by one or more content delivery nodes (120, 130) of a content delivery system; responsive to the domain name lookup request, identifying a network performance profile (111, 311, 312) for the end user device (140, 340-344) based on an internet service provider associated with the end user device (140, 340-344), wherein the network performance profile (111, 311, 312) is created based on network characteristics obtained from at least a second end user device (140, 340-344) that shares the internet service provider with the end user device (140, 340-344), and wherein the network characteristics comprise performance measurements, being made by the at least the second end user device of data transferred between the at least the second end user device (140, 340-344) and at least one of the content delivery nodes (120, 130); selecting a content delivery node (120, 130) of the content delivery system based on the network performance profile; and transferring a network address (121, 131, 615) associated with the selected content delivery node (120, 130) for receipt by the end user device (140, 340-344) responsive to the domain name lookup request; wherein identifying the network performance profile (111, 311, 312) for the end user device (140, 340-344) based on the internet service provider associated with the end user device (140, 340-344) comprises identifying the network performance profile (111, 311, 312) for the end user device (140, 340-344) based on a network address range containing a network address of the end user device (140, 340-344).