My opinion is that when you're dependent upon a (possibly recalcitrant) upstream provider, it's best to have means of dealing with whatever they give you. If i

My opinion is that when you're dependent upon a (possibly recalcitrant) upstream provider, it's best to have means of dealing with whatever they give you. If in some circumstance someone hands you only one address (or, one address too few, perhaps), you can still hang your own network behind it.