mac地址是怎么变的?

我的软件有一个相当粗糙的许可证密钥，它可以找到一台机器的所有mac地址并对它们进行加密，然后将它们与注册表中保存的加密值进行比较。如果注册表中的值与我刚刚找到的任何值匹配，则软件将加载。我希望我总是得到相同的MAC地址值，但对于一些机器，这不是真的。谁能解释为什么他们的MAC地址不是固定的，或者如何克服这个问题?

        //// GET MAC ADDRESS
        String sMacAddress = String.Empty;
        ManagementScope theScope = new ManagementScope("''''" +Environment.MachineName + "''root''cimv2");
        DGCSLogger.log.Trace("Management scope");
        StringBuilder theQueryBuilder = new StringBuilder();
        theQueryBuilder.Append("SELECT MACAddress FROM Win32_NetworkAdapter");
        ObjectQuery theQuery = new ObjectQuery(theQueryBuilder.ToString());
        DGCSLogger.log.Trace("Creating searcher");
        ManagementObjectSearcher theSearcher = new ManagementObjectSearcher(theScope, theQuery);
        DGCSLogger.log.Trace("Creating collection");
        ManagementObjectCollection theCollectionOfResults = theSearcher.Get();
        DGCSLogger.log.Trace("Got management objects");
        //// GET VALUE FROM REGISTRY
        String sSavedScrambled = RegistryHelper.GetCurrentUserValue(@"Software'VB and VBA Program Settings'FMPos'settings", "LicenceKey");
        String sSavedLicenceKey = String.Empty;
        if (sSavedScrambled != null)
            sSavedLicenceKey = DGCS.Common.Password.UnScramblePassword(sSavedScrambled);
        String sMacNoLicenceKey = String.Empty;
        String sMacNo  = String.Empty;

         ///// COMPARE VALUES WITH REGISTRY VALUE
        foreach (ManagementObject theCurrentObject in theCollectionOfResults)
        {
            DGCSLogger.log.Trace("foreach object");
            if (theCurrentObject["MACAddress"] != null)
            {
                DGCSLogger.log.Trace("foreach object: " + theCurrentObject["MACAddress"].ToString().Substring(0, 5));
                String macAdd =  theCurrentObject["MACAddress"].ToString();
                sMacNo = DGCS.Common.Password.ScrambleMacNumber(theCurrentObject["MACAddress"].ToString());
                sMacNoLicenceKey = DGCS.Common.Password.CreateMacNoPassword(sMacNo);
                DGCSLogger.log.Trace(": " + theCurrentObject["MACAddress"].ToString().Substring(0, 5));
                if (sMacNoLicenceKey.Trim() == sSavedLicenceKey.Trim())
                    lkCheck.HasLicenceKey = true;
            }
        }