Further, we can assert that DNS supports mappings of this scale and does soin a caching fashion. However, DNS is NOT invoked for anonymous transactions and if it was, it would not scale appropriately. ImagineGoogle doing a reverse DNS lookup on every packet it receives, for example.
I believe a site will have less Map-Requests sent by the ITRs then DNS queries by all hosts at the site. The granularity of site reduces the number of requests and are reused by all other hosts that come later.
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