Why is `to.` a valid domain name?

In visiting http://to./ you are given a legitimate website.

Is to. a valid domain name then, despite not ending with a TLD and having a superfluous period? 开发者_Go百科Why?

Being valid, what would its DNS hierarchy be?

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The final dot is part of the fully qualified domain name. More information in this article. Specifically:

It's a little-known fact, but fully-qualified (unambiguous) DNS domain names have a dot at the end. People running DNS servers usually know this (if you miss the trailing dots out, your DNS configuration is unlikely to work) but the general public usually doesn't. A domain name that doesn't have a dot at the end is not fully-qualified and is potentially ambiguous.

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to is the TLD of Tonga.

There is no spec that says that a domain name must have something other than a TLD; Tonga is the only TLD that has an A record for the TLD itself.

However, most browsers will not recognize a domain name that doesn't contain a period, so they use the full FQDN, with a trailing ..

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The DNS represents a hierarchy of domain names. As T. pointed out, if you see a dot at the end of a FQDN, it just represents the upper root point of the whole domain name tree.

In the context of web browsers, they tend to be graceful and hide this detail from end users.