[lch]

Please say what operating system you are on. I assume it is some kind of Windows? Which version?

There may be firewalls that you are not aware of. Anti virus programs sometimes install one, Microsoft's Internet Connection Sharing is one and on top of that it could be that the network settings are making trouble. I am not so sure what Windows does exactly when somebody creates a "network bridge" in the network settings, which some people like to do because Windows gives them that option, or maybe it is even added automatically, but I know that on our network lots of problems have been solved by just asking the users to delete that network bridge. More info please.