Abstract: A method of authenticating a content-provider server, the method comprising: determining a domain name of the content-provider server; obtaining a fragment of a database of IP addresses, the fragment corresponding to the domain name of the content-provider server and storing one or more IP addresses associated with the domain name; comparing the IP address of the content-provider server against the IP addresses of the fragment; and providing an indication that the IP address of the content-provider server is included or excluded from the fragment of IP addresses. Additionally, a client computer and server operable to implement the method are described.