Win98 map network drive of a Win2K, initial success, but fail later...Error 53

1. Windows 98 clients form a workgroup.
2. These Win98 use net use command to map network drive of a Windows 2000
machine (a workstation), which is located in the internet.

e.g. net use Z: \\10.10.10.10\public password
(of course, a same local user is created in these 2 computer)

3. After a period of time, may be within 1 hour, the mapped network drive
will be disconnected. If users re-map the same network drive, they will get
error 53: "the network path was not found". The drive can only be re-mapped
after system restart.


Why this situation exists?? Who can provide a solution for me??