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Function to Verify if Connected to the Internet

  • May 15, 2000
  • By Dmitri Utenkov
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As many of you are aware, there is already an article on CodeGuru, 'BOOL IsInternetConnected()' by Ran, that contains a function that checks for Internet connectivity. However, while Ran's version of this function is very good, in my experience I found that it wasn't the best for my applications. One of the problems that I ran into was that the application loses focus while the function attempts to check for the Internet connection. To that extent, I present to you this alternative.

In addition, I'd like to add that this function is based on the comments made on this site by Jarek Gibek as well as the article 'Get hostname and ip address of local computer(2)' by Jaroslav Pisk.

How it Works

The way this function works is that it attempts to define which are the IP addresses which can automatically be excluded from consideration. Then it defines which IP addresses would signify that the computer is connected to the Internet. Here's a sampling of IP addresses and what they mean to the function that should help to illustrate my point.
  • - localhost;
  • Private Adress area(RFC 1597)
    • 10.*.*.* - Class A;
    • 172.16-31.*.* - Class B;
    • 192.168.*.*- Class C;
  • 169.254.*.* Microsoft AutoNet address area(See: http://lancanyon.com/autonet.htm for detail);
  • Have you reason for other ranges exclusion?
Please note that I do not exclude other specific IP adresses such as or 255.2555.255.255 because the local host can't have either of these IP addresses.

The Function

While the function is stated (and commented) here, you can also download it below.
#include "winsock2.h"

BOOL IsInternetConnection()
 char szHostName[128];

 BOOL bPrivateAdr = false; // Private Address area
 BOOL bClassA = false;     // Class A definition
 BOOL bClassB = false;     // Class B definition
 BOOL bClassC = false;     // Class C definition
 BOOL bAutoNet = false;    // AutoNet definition
 CString str;

 if (gethostname(szHostName, 128) == 0 )
  // Get host adresses
  struct hostent * pHost;
  int i;
  UINT ipb;
  pHost = gethostbyname(szHostName); 

  for (i = 0; 
       pHost!= NULL && pHost->h_addr_list[i]!= NULL; 
       i++ )
   int j;
   bClassA = bClassB = bClassC = false;
   for( j = 0; j < pHost->h_length; j++ )
    CString addr;

    if( j > 0 )	str += ".";
    ipb = (unsigned int)((unsigned char*)pHost->h_addr_list[i])[j];

    // Define the IP range for exclusion
     if(bClassA = (ipb == 10)) break; // Class A defined
     bClassB = (ipb == 172); 
     bClassC = (ipb == 192);
     bAutoNet = (ipb == 169);
    else if (j==1)
     // Class B defined
     if(bClassB = (bClassB && ipb >= 16 && ipb <= 31)) break;

     // Class C defined
     if(bClassC = (bClassC && ipb == 168)) break;

     //AutoNet pasibility defined
     if(bAutoNet = (bAutoNet && ipb == 254)) break;

    addr.Format("%u", ipb );
    str += addr;
   // If any address of Private Address 
   // area has been found bPrivateAdr = true
   bPrivateAdr = bPrivateAdr || bClassA || bClassB || bClassC;

   // If any address of Internet Address area 
   // has been found returns TRUE
   if (!bClassA 
   && !bClassB 
   && !bClassC 
   && !bAutoNet 
   && str != "" 
   && !str.IsEmpty()) 
    return TRUE;

 if (bPrivateAdr)
  // The system has IP address from Private Address 
  // area,only. Internet from the computer can be accessable 
  // via Proxy. If user turn on proxy connection flag, the 
  // function believe Internet accessable.
  return bProxyConnection;
 return false;


Download source (same function as listed above) - 1 Kb

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