A method comprising: selecting, by an auction server, based on ad requests received from one or more advertisers or bids received from the one or more advertisers for previously provided bid requests, and based at least in part on amounts of received bids, an ad request associated with an ad tag, the ad tag identifying a location associated with an advertiser where advertisement content associated with the ad request is stored by specifying a network address of the location associated with the advertiser, wherein the advertisement content is advertisement content for presentation in an ad space in content provided or managed by a web server including at least one of code and a network address communicating with the auction server for retrieving the advertisement content, receiving, by an ad proxy server, the ad tag from the auction server; retrieving, by the ad proxy server the advertisement content associated with the ad tag from the location specified by the network address; storing a copy of the retrieved advertisement content in a location (150,180) not associated with the advertiser; generating, by the ad proxy server a modified ad tag from the received ad tag, the modified ad tag specifying a network address of the location (150,180) not associated with the advertiser instead of a location associated with the advertiser; sending the modified ad tag to a client device requesting presentation of an advertisement; receiving a request to present the copy of the retrieved advertisement content from the client device based on the modified ad tag at the location not associated with the advertiser specified by the network address of the modified ad tag; and sending the stored copy of the retrieved advertisement content to the client device. selecting, by an auction server, based on ad requests received from one or more advertisers or bids received from the one or more advertisers for previously provided bid requests, and based at least in part on amounts of received bids, an ad request associated with an ad tag, the ad tag identifying a location associated with an advertiser where advertisement content associated with the ad request is stored by specifying a network address of the location associated with the advertiser, wherein the advertisement content is advertisement content for presentation in an ad space in content provided or managed by a web server including at least one of code and a network address communicating with the auction server for retrieving the advertisement content, receiving, by an ad proxy server, the ad tag from the auction server; retrieving, by the ad proxy server the advertisement content associated with the ad tag from the location specified by the network address; storing a copy of the retrieved advertisement content in a location (150,180) not associated with the advertiser; generating, by the ad proxy server a modified ad tag from the received ad tag, the modified ad tag specifying a network address of the location (150,180) not associated with the advertiser instead of a location associated with the advertiser; sending the modified ad tag to a client device requesting presentation of an advertisement; receiving a request to present the copy of the retrieved advertisement content from the client device based on the modified ad tag at the location not associated with the advertiser specified by the network address of the modified ad tag; and sending the stored copy of the retrieved advertisement content to the client device.