You have connected via network or modem to an NT4 or Windows 2000 system (Server or Workstation/Professional), you are not using the connection
for a while (having an "idle" connection: no network traffic) and you get
automatically disconnected from the Server.

That can be caused by an entry in the Registry, there are different entries
for LAN connections and RAS connections:

LAN(See also Q138365):
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\LanmanServer\Parameters:
key: "Autodisconnect", the value is defined in minutes.

Note: a value of "-1" disables AutoDisconnect, a value of "0" does NOT
turn it off and results in very fast disconnects within a few seconds of idle time.
(However, the RAS Autodisconnect paramater is turned off if you set it to a value of 0.)

Note: the number of minutes can be set at a command prompt using the
"NET CONFIG SERVER" command.

Example: to disconnect after 30 min:

To turn off the automatic disconnection, use as value: -1:

However, according to MS, it is preferable to modify the Autodisconnect entry
in the registry, since a modification via command-line (as above) may turn off
its autotuning functions (more info in Q128167: Server Service Configuration and Tuning)

RAS (See also Q153944 andQ181431 )
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\RemoteAccess\Parameters:
key: "Autodisconnect", the REG_DWORD value is defined in min, default:20.