1. A content delivery system comprising: a first point of presence (“POP”) comprising a first plurality of computing devices, the first POP configured to retrieve and respond to client requests for a plurality of sets of content; a second POP comprising a second plurality of computing devices, the second POP configured to retrieve and respond to client requests for the plurality of sets of content; and one or more computing devices implementing an attack mitigation service, the one or more computing devices configured with specific computer-executable instructions to: detect a network attack on the first POP, wherein the network attack is directed to a combination of network addresses utilized by the first POP; identify, based at least in part on the combination of network addresses, a first set of content, from the plurality of sets of content, as a target of the network attack; identify, based at least in part on the combination of network addresses, a second set of content, from the plurality of sets of content, as not targeted by the network attack, wherein the second set of content is made available at at least one network address of the combination of network addresses; disassociate the second set of content from the at least one network address; and modify routing of network transmissions to the first set of content based at least partly on transmitting instructions to the first POP to withdrawal its association with the combination of network addresses and transmitting instructions to the second POP to generate an association between the second POP and the combination of network addresses.