#include <afxinet.h>

//GB2312 转换成 Unicode
wchar_t* GB2312ToUnicode(const char* szGBString)
{
    UINT nCodePage = 936; //GB2312

    int nLength=MultiByteToWideChar(nCodePage,0,szGBString,-1,NULL,0);

    wchar_t* pBuffer = new wchar_t[nLength+1];

    MultiByteToWideChar(nCodePage,0,szGBString,-1,pBuffer,nLength);

    pBuffer[nLength]=0;

    return pBuffer;
}

void CNetDlg::OnGetNetIp()
{
    // 本地化，以支持中文
    _tsetlocale(LC_ALL, _T("chs"));   // #include <locale.h>
    CString strSource;
    char chSource[4096] = {0};
    CString Address;
    CInternetSession mySession(NULL,0);
    CHttpFile* myHttpFile=NULL;

    Address=_T("http://20140507.ip138.com/ic.asp");//ip138网页

    myHttpFile=(CHttpFile*)mySession.OpenURL(Address);//读取网络地址

    try{
        while(myHttpFile->Read(chSource, 4096))
        {  
            //循环读取下载来的网页文本
            int begin=0;

            WCHAR* wchSource = GB2312ToUnicode(chSource);
            strSource = wchSource;
            delete[] wchSource;
            OutputDebugString(strSource);

            begin=strSource.Find(_T("["), 0);

            if(begin!=-1)//如果找到"["， 则找"]"  中括号内的文本则是 你的外网ip
            {
                int end=strSource.Find(_T("]"));
                CString m_internetip=strSource.Mid(begin+1,end-begin-1);//提取外网ip

                AfxMessageBox(m_internetip);
                break;
            }
        }
    }
    catch(CInternetException *e)
    {
        AfxMessageBox(e->m_dwContext);
    }
}

参考：

http://blog.csdn.net/cbk861110/article/details/7844729

http://www.ip138.com

的网页源码中查找到。

<tr><td align="center"><h3>www.ip138.com IP查询(搜索IP地址的地理位置)</h3></td></tr>
	<tr>
		<td height="30" align="center" valign="top"><iframe src="http://20140507.ip138.com/ic.asp" rel="nofollow" frameborder="0" scrolling="no" width="100%" height="100%"></iframe></td>
	</tr>

MFC C++ 获取外网IP地址