6.7. How do I tell if a machine is connected to the Internet?

Some techniques include :- IsNetworkAlive()
If you are targeting systems with IE5 an 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. 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 "RasEnumConnectionsA" from "RASAPI32.DLL", as LAN installations of Windows may not include the library. InternetGetConnectedState()
This a 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 off line flag changed for IE5 -it returns TRUE for connected' even when off line, but signals the flag in the LPDWORD parameter. InternetCheckConnection()
A Wininet/IE4 function call. This is meant to determine if a URL is reachable -in practice it is pretty unreliable and best avoided. 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 new function calls WM_DEVICECHANGE messages, which tell your app when a slow or fast network adapter has arrived or departed. There is no way to tell if the adapters are actually connected to anything though. NT4 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. 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.