URLs always have this format:


The problem is that IPv6 uses colons, just like the separator of port and host, e.g:


But what if this is the host, and I want to connect to it with HTTP on port 100?


The problem is the last colon. Since zero's are omitted with double colons (between 1f70 and 999), it's unknown if ':100' belongs to the IP or the port number. How can we know this?


The notation in that case is to encode the IPv6 IP number in square brackets:


That's RFC 3986, section 3.2.2: Host

A host identified by an Internet Protocol literal address, version 6 [RFC3513] or later, is distinguished by enclosing the IP literal within square brackets ("[" and "]"). This is the only place where square bracket characters are allowed in the URI syntax. In anticipation of future, as-yet-undefined IP literal address formats, an implementation may use an optional version flag to indicate such a format explicitly rather than rely on heuristic determination.

  • 43
    It's worth pointing out that the brackets are not optional. It's the only unambiguous mechanism by which the browser can identify a numeric IPv6 address. – tylerl Nov 25 '10 at 3:58
  • Shouldn't the port be inside the brackets? – jayarjo Dec 20 '16 at 15:30
  • 3
    @jayarjo No, as the brackets are there to provide disambiguation between the IP address, which contains colons, and the port, which is separated from the IP address by a colon. – sysadmin1138 Dec 22 '16 at 13:26

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