A method for detecting an IP address. An access router releases the routes of a detected IP address via a first interface and a second interface, and sets the priority of the route released via said first interface lower than that of the route released via said second interface. After having received by said access router a WEB request by the user via said first interface, the method comprises searching a session record corresponding to said WEB request; establishing by said access router the session record corresponding to said WEB request when the corresponding session record is not found, and returning a redirecting message taking said detected IP address as the re-directed destination IP address; and after having received by said access router via said second interface the message taking said detected IP address as the destination IP address, determining that the user who sent said message is an illegal user.