How can a webservice find out the URL the client used to call this webmethod?

Suppose i have a webservice (http://myWebservice/CodeGenerator/CodeValidator.asmx) as follows

[WebMethod]
public string ValidateCode(string sCode)
{
    // I need 开发者_高级运维to check requested url here. The call comes from different sites.
    // For example www.abc.com/accesscode.aspx. I need to get this url. 
}