How is http://d§.cc/ a valid URL?

Question

While browsing online today, I came across the following URL:

http://d§.cc/

Somehow it works. In Firefox 3.6/Mac, the browser seems to interpret the URL as:

http://xn--d-jca.cc/

In Safari 5/Mac, the URL does not change when I navigate to it.

I'd always understood that the § character (section sign), among others, is invalid in URLs. To quote RFC 1738:

Thus, only alphanumerics, the special characters "$-_.+!*'(),", and reserved characters used for their reserved purposes may be used unencoded within a URL.

Is this an oddity of character encoding? And, how/where can I register my own § URL?

Answer 1

This is an Internationalized domain name, or IDN. The encoding it uses is called punycode.

Many big registrars are in the business of selling IDN domain names, including GoDaddy, but when I registered my vanity IDN (ə.tv) I found Name.com to be easier. They cost the same as "regular" domains.

Here's a list of valid IDN characters--beware, however, that many of these characters won't display correctly in all fonts, so if you're in the market for an IDN make sure you test it in commonly-used fonts on your target platforms before making a purchase.

Answer 2

This is called an IDN (Internationalized Domain Name). It has nothing to do with URIs, it is a feature of the DNS (Domain Name System) which resolves human-readable DNS names such as www.example.com into IP addresses such as 127.0.0.1.

Because originally the Domain Name System was ASCII-only, IDN was introduced in a backwards-compatible manner: DNS names that contain non-ASCII characters are encoded into DNS names that contain only ASCII characters using an algorithm called Punycode which is specifically designed to be efficient for the types of strings that are typically contained in a DNS name and still be somewhat human-readable even in the encoded form.

The main purpose of IDN is, as the name implies, to make it possible to register domain names in languages other than English. However, a side effect is that now all Unicode characters including symbols that don't really have anything to do with non-English languages can be used in domain names.