Actually, there is no single function for determining if a machine is connected to the Internet, and it is impossible to reliably determine what is happening without side effects - such as automatic network connections taking place. What you can do is reliably detect when there definitely isn't an Internet Link: in the absence of any dial up or LAN connection the system is definitely off line.
Some techniques include:
1. IsNetworkAlive() If you are targeting system with IE5 or later, this is the best API call yet it even listens for traffic on a LAN. There is a secondary function IsDestinationReachable() which tries to resolve the hostname and ping it. This does not work through firewalls, and overestimates speed as the max the LAN card can support, rather than the actual point to point bandwidth.
2. RasEnumConnections() A reliable technique for modems and direct dial up networking, but not for situations where Internet access is via a LAN. You should dynamically load "RasEnumConnectionA" from "RASAPI32.DLL", as LAN installations of Windows may not include the library.
3. InternetGetConnectedState() This Wininet /IE4 function call can distinguish between modem and LAN, but can't handle complex LAN+autodial router situations. It is "offline state aware". Important: handling of the offline flage changed for IE5 -it returns TRUE for connected' even when off line, but signals the flags in the LPDWORD parameter.
4. InternetCheckConnection() A Winnet/IE4 function call. This is meant to determine if a URL is reachable- in practice it is pretty unreliable and best voided.
5. NT SP4, NT5: The IP helper API can tell you which network interface to use to connect to a supplied IP address, and what the bandwidth and current status of that link is
6. Using the Offline flag which is part of IE4 to allow users to manually control the online/offline state of applications. This flag is stored in the registry and can be manipulated via some funcions calls These calls mostly determine the presence or absence of network connections -not Internet access, so can't handle a home network sharing a dial up connection, or two laptops connected directly to each other. The global offline state flag of IE4 (and hence win98, NT5) and the call to test it - InternetGetConnectedState()- look the best long term options, but will take time to become universal. The IP Helper APIs even let you find out how much traffic is going over a link, but only detect the 'loopback' interface on Windows 98, so is not a lot of use. Wouldn't a 'GetSpeedToHost() function call be great?
Finally, whatever technique you use, when it's time to talk to a remote site, always add timeouts or a cancel button. Even a quick functions like gethostbyname() can lock up an app if something in the network chain is broken.
ref:
How to test the reachability of a VPN-Connection? - http://www.codeproject.com/KB/IP/SimpleLineTester.aspx