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IsNetworkAlive Function
IsNetworkAlive Function
 
The IsNetworkAlive function determines whether or not a local system is connected to a network, and identifies the type of network connection, for example, a LAN, WAN, or both.
 

Bool IsNetworkAlive(
  __out         LPDWORD lpdwFlags


);

Parameters
lpdwFlags
The type of network connection that is available. This parameter can be one of the following values:
 
NETWORK_ALIVE_LAN
The computer has one or more LAN cards that are active.
NETWORK_ALIVE_WAN
The computer has one or more active RAS connections.
NETWORK_ALIVE_AOL
This flag is only valid in Windows Me/98/95.
Indicates a computer is connected to the America Online (AOL) network.
Return Value
This function returns one of the following values:
Return code
Description
TRUE
A local system is connected to a network. For information about the type of connection, see the lpdwFlags parameter.
FALSE
There is no connection. For extended error information, call GetLastError .
Remarks
This function can be used by an application to determine whether or not there is network connectivity before proceeding with network operations. A directory service type of application, e-mail client, or Internet browser can adapt to various types of network connectivity. For example, a printing operation can be deferred until a network connection is available.
It may not always be practical for an application to call IsNetworkAlive to determine whether or not a local system is disconnected from a LAN, because IsNetworkAlive can be slow, and it may take too much time for the function to detect that a local system is disconnected. However, IsNetworkAlive can always identify a WAN connectivity at the moment.
Note   This function is only available for TCP/IP connections.
また、ここを見てもいいです.
http://blog.csdn.net/nitghost/archive/2009/02/08/3868439.aspx