Check UserName availability in MasterPage using jQuery AJAX in ASP.Net

Actually in ur code for client side function :

function ShowAvailability() {
    $.ajax({
        type: "POST",
        url: "CS.aspx/CheckUserName",
        data: '{userName: "' + $("#<%=txtUserName.ClientID%>")[0].value + '" }',
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: OnSuccess,
        failure: function(response) {
            alert(response);
        }
    });
}

you mentioned url .. how to handle this from master page.. as it is common for all pages.

i want to put this code you mentioned in example at masterpage.