1. A method for categorizing URLs (Uniform Resource Locators) of web pages accessed by users over an IP (Internet Protocol) based data network, the method comprising: collecting by means of at least one monitoring probe real time data from IP data traffic occurring on the IP based data network; extracting from said collected real time data parameters related to a web page, said parameters including an URL of the web page; processing said URL with a rule based categorization engine, to associate a matching category to the URL of said web page, the matching category being inferred from a pre-defined list of categories; when no matching category is inferred, transferring said URL of said web page to a semantic based categorization engine; and processing said transferred URL by the semantic based categorization engine, said processing consisting in: extracting textual content from said web page associated to said URL, performing a semantic analysis of said textual content, and associating a matching category to the transferred URL of the web page based on the semantic analysis of the textual content extracted from the web page, the matching category being inferred from a pre-defined list of categories.