public void AddDivMessage()
{
    Uri currentUri = new Uri(webBrowser1.Url.ToString());
    String hostName = null;

    // Ensure we have a host name, and not just an IP, against which to test.
    if (!(currentUri.HostNameType == UriHostNameType.Dns))
    {
        DnsPermission permit = new DnsPermission(System.Security.Permissions.PermissionState.Unrestricted);
        permit.Assert();

        IPHostEntry hostEntry = System.Net.Dns.GetHostEntry(currentUri.Host);
        hostName = hostEntry.HostName;
    } else {
        hostName = currentUri.Host;
    }

    if (!hostName.Contains("adatum.com"))
    {
        AddTopPageMessage("You are viewing a web site other than ADatum.com. " +
            "Please exercise caution, and ensure your Web surfing complies with all " +
            "corporate regulations as laid out in the company handbook.");
    }
}

private void AddTopPageMessage(String message)
{
    if (webBrowser1.Document != null) 
    {
        HtmlDocument doc = webBrowser1.Document;

        // Do not insert the warning again if it already exists. 
        HtmlElementCollection returnedElems = doc.All.GetElementsByName("ADatumWarningDiv");
        if ((returnedElems != null) && (returnedElems.Count > 0)) 
        {
            return;
        }

        HtmlElement divElem = doc.CreateElement("DIV");
        divElem.Name = "ADatumWarningDiv";
        divElem.Style = "background-color:black;color:white;font-weight:bold;width:100%;";
        divElem.InnerText = message;
        
        divElem = doc.Body.InsertAdjacentElement(HtmlElementInsertionOrientation.AfterBegin, divElem);
    }
}