I agree with everything Роман Донченко wrote.
I just wanted to comment these two things:
4.2. "::" Usage 4.2.1. shorten as much as possible 4.2.2. One 16 bit 0 Field 4.2.3. When "::" Can Be Used Twice
When cases where it is possible to use "::" in two or more different sections of an address, implementation to shorten the side with longer 16 bit 0 fields are more common (i.e. latter is shortened in 2001:0:0:1:0:0:0:1). When the length of 16 bit 0 fields are equal (i.e. 2001:db8:0:0:1:0:0:1), the former is usually shortened.
The wordings "can be used twice" and "is possible to use ..." are IMO misleading, since :: cannot be used twice.
Maybe you should use something like "when there are two or more alternative placings for ::"?
-- Aleksi Suhonen -------------------------------------------------------------------- IETF IPv6 working group mailing list ipv6@ietf.org Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------