Under IPv4, the de facto standard notation is completely clear: it is an IP address with port number. This is understood in URL's for instance:

If I have a sockaddr_ipv4 class in some code, its tostring method can return this colon notation and everything is cool.

In a fit of myopia, the designers for the IPv6 addresses numeric notation decided that the colon character was available for their use as a separator. As a result, a colon-delimited port number looks ambiguous on an IPv6 address.

How can we incorporate a port number into a printed representation of an IPv6 address-with-port? Is there some de facto standard way?

  • 5
    Boom and bam
    – Ordous
    Feb 29, 2016 at 16:25
  • 1
    I think what @Ordous is trying to say is that this question is a duplicate of some existing questions with answers :) Feb 29, 2016 at 16:27
  • @Ordous ...and, of course, bang
    – gnat
    Feb 29, 2016 at 16:28
  • 3
    It's not a duplicate of any question here; those are off topic on those sites. The question isn't about a broken piece of code, so it's not appropriate for StackOverflow, and it's not a sysadmin issue for ServerFault. It's a "conceptual questions about software development": what do I put into the tostring method of an IPv6 socket address class.
    – Kaz
    Feb 29, 2016 at 16:34

1 Answer 1


The common way of doing this is to enclose the IPv6 address in square brackets like this:


This is what is being used in URLs for example:

  • 3
    You forgot to copy over the RFC reference from the SO answer. :)
    – Kaz
    Feb 29, 2016 at 16:34
  • In fact RFC 5952 has a more well-rounded answer in section 6. The [ADDR]:PORT isn't the only notation; just the one that should be used in URLs according to RFC 3986.
    – Kaz
    Feb 29, 2016 at 20:11
  • For instance, it turns out that this is acceptable, for an IPv6 mapped IPv6: ::ffff:, without the square brackets. (Though that is not valid in URL use, it is not ambiguous.)
    – Kaz
    Feb 29, 2016 at 22:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.