The Windows operating systems have distinct methodologies for designing and implementing networks, and have specific systems to accomplish various networking processes, such as Exchange for email, Sharepoint for shared files and programs, and IIS for delivery of web pages. Microsoft also produces server technologies for networked database use, security and virtualization.
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Make sure the client machines pint only to your internal DNS server (assuming you have servers) and not the ISP even as an alternate.
Another thought is many commercial routers have # of connection limits with 10 being the entry level. If an outsider with a laptop connects it will remember that MAC address. Restarting the router resets the counter. However this would not affect internal connections.