You need to identify which machine that is and find out why it is trying to access your server. The -a switch in ping will return the machine name if it exists. You may be able to use that to identify it.
You are getting this message because of an illegal access attempt via Networker. It is no different than someone trying to access your domain illegally. You should respond based upon whatyou find. It may be an innocent attempt to access the server, or something else. What that is, only you can determine.