Question

I cannot find the reason why IPv6 use colon as delimiter instead of dot.
When address is described with port number, it seems clumsy to me.

[2001:db8:85a3:8d3:1319:8a2e:370:7348]:443

What justify using colon?

Answer 1

I looked through the older IPv6 RFCs in order to find a clue about the history, but I think the answer lies in RFC 4291 section 2.2.3:

3. An alternative form that is sometimes more convenient when dealing
   with a mixed environment of IPv4 and IPv6 nodes is
   x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadecimal values of
   the six high-order 16-bit pieces of the address, and the 'd's are
   the decimal values of the four low-order 8-bit pieces of the
   address (standard IPv4 representation).  Examples:

      0:0:0:0:0:0:13.1.68.3

      0:0:0:0:0:FFFF:129.144.52.38

   or in compressed form:

      ::13.1.68.3

That is, you wouldn’t be able "embed" an IPv4-compatible address if IPv6 addresses were dot-delimited.

To address your implication about the colon clashing with a port number in a URL, I think the answer is that at the time that IPv6 was being designed in the early 1990s, hypertext and the URL were being designed at the same time. Since port numbers are separate from IP addresses, there was no standard for specifying a (port, address) tuple. (The command-line telnet client, for example, uses a space to separate the IP address from the port.)