I don't think that the DNS system would count as prior art which could invalidate 6,199,048, because it has nothing to do with scanning reflected light off of something based on a standardized pattern. Patents very frequently build off of related ideas; in fact, this patent specifically references URLs, which are built on top of the domain name system, so it was obvious at the time of examination that it was based on DNS.
In order to be patentable, an invention doesn't have to be completely out of the blue with no basis in prior inventions. It just has to introduce some novel element, that hasn't been invented before. So, while DNS had existed before, it would have been possible to patent the URL, which builds on DNS but adds several more features. Or likewise, it's possible to patent something that builds on the existing inventions, of a URL and a bar code.
Now, many of us in the tech community would say that that is something that's obvious to someone skilled in the art; combining a URL and a bar code takes no real skill at at all. But I think that many of us in the tech industry are at odds with the current stance of the patent office and jurisprudence on the topic, so that doesn't help you very much.