So I think in this instance I would deem valid to be: 1) the url is the correct format 2) the url resolves to an ip address 3) the url is registered and is in u

So I think in this instance I would deem valid to be: 1) the url is the correct format 2) the url resolves to an ip address 3) the url is registered and is in use. By this I mean it's not one of the "this domain name is for sale" pages. Number 3 is the novel and challenging piece of this.