1. A method of collecting addresses for remotely accessible information sources for use in updating a filtering system ( 100 ) which controls access to such information sources via a public access network (N 1 ), the method comprising; interrupting transmission of a message destined for a terminal (T 2 , T 3 ) other than the filtering system ( 100 ); identifying an address for a remotely accessible information source from the message; analyzing the message on the basis of an attribute of the message so as to classify the message as either a first type of message or a second, different, type of message; and if the message is classified as the first type of message, transmitting a filter message comprising data indicative of the identified address to the filtering system ( 100 ).