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The 03 NetBIOS extension typically indicates that the name was registered by the Messenger service. During boot, as the Messenger service starts, it will attempt to register 'machinename 03'. When a user logs on, the Messenger service will attempt to register 'username 03'. Since these names are declared as unique (as opposed to being group names), only one machine in each NetBIOS name space can register each name. If the name is already registered by another machine, the registration attempt will fail.
Theoretically, these names are only used by the Messenger service. For example, if you run the command 'net send backup Test message', the messenger service will attempt to resolve 'jdoe 03' to an IP, then it will send a NetBIOS datagram to the NetBIOS name 'jdoe 03' at that IP.
If the Messenger service of the target machine has successfully registered 'jdoe 03', it will accept the datagram and open a pop-up dialog box containing the message ("Test message").
If the messenger service of the target machine has not successfully registered this name, the datagram will be ignored.
The name conflict with 03 names are common on networks where a user is logged onto multiple machines at the same time or where machines and user accounts have the same names. The only effect is the loss the ability to receive Messenger service datagrams on that name, by the machine that failed to register the name.
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