如何在SharePoint上查找用户的IP地址

我有一个Web部件,它通过一个简单的按钮向SharePoint上的用户显示。单击按钮后,我会记录单击该按钮的用户、时间和他们的IP地址。我搞不懂的是如何找到他们的IP地址?我可以通过SharePoint对象模型实现它,还是必须做一些更复杂的事情?

private bool SignInCurrentUser()
        {
            SPWeb web = SPContext.Current.Web;
            SPUser user = web.CurrentUser;
            String address = "?";

            SPList regList = web.Lists["SEED MEng Lab Registration List"];

            SPListItem registration = regList.Items.Add();
            registration["Student"] = user;
            registration["Occurrence"] = DateTime.Now;
            registration["IP Address"] = address;
            registration.Update();

            return true;
        }