Web server 101 receives a request for a web page from a client, identifies that the webpage includes content requiring the client to be provided with an identifier, generates the identifier and provides it to the client. The identifier includes data that changes from an identifier generated in response to one client request to an identifier generated in response to another client request. On determination that the client has not requested the resource using the identifier, access to a website associated with the web page is restricted, possibly until the client prevents a filter from blocking resources being downloaded or makes a subscription payment. The identifier may include an address associated with a source of the web page or a data string that is formed by encrypting a Universal Resource Locator or address associated with the resource. Links may be embedded within e.g. an online newspaper web page that trigger the client to request advertising content from third party provider 104. May use addresses that are not identifiable to an ad blocker or client-side filter to circumvent software which recognises URLs and HTML elements that are associated with adverts or browsing trackers embedded within web pages.
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