There are several programs that are used to connect to our network that will automatically assign a chatter a "guest" nickname if they do not choose a nick of their own. Our .de portal uses WebChatter_1111 (where 1111 can be any random number). LightIRC uses LightIRC_1111, and some connections will get a Guest_1111 type nick. These are generally visitors who are just checking things out. Some of them are willing to chat, and some won't answer, but as long as they are not violating any rules here, they are nothing to worry about.
TNTease