Invisible IPv6 addresses

[Celejar <celejar@gmail.com>]

Hi,

[Somewhat off-topic, since this is a general networking topic:]

I have Verizon Fios residential internet service. Verizon is
notoriously cagey about its deployment of IPv6 for residential service
[1], but I don't seem to officially have it - my router (running
OpenWrt) tries dhcp6 but fails to get anything, and all the router's
interfaces show only link-local addresses for IPv6.

I noticed, however, that when I try to connect to the router over the
public internet via a DDNS hostname tied to the router's external IPv4
address, the connections take place via IPv6! I dug (no pun intended) a
bit further, and found this (where a.b.c.d is my Verizon-assigned (via
dhcp) dotted-quad IPv4 address):

~$ host a.b.c.d
d.c.b.a.in-addr.arpa domain name pointer pool-a-b-c-d.region.fios.verizon.net.

(where 'region' is my geographic region).

~$ host pool-a-b-c-d.region.fios.verizon.net
pool-a-b-c-d.region.fios.verizon.net has address a.b.c.d
pool-a-b-c-d.region.fios.verizon.net has IPv6 address e:f:g:h:i:j:k:l

So lo and behold, I do seem to have an IPv6 address, and this address
e:f:g:h:i:j:k:l is indeed the one that my clients are using to reach
the router (xxx.yyy.zzz is the DDNS hostname):

~$ ping xxx.yyy.zzz
PING xxx.yyy.zzz(pool-a-b-c-d.region.fios.verizon.net (e:f:g:h:i:j:k:l)) 56 data bytes
64 bytes from pool-a-b-c-d.region.fios.verizon.net (e:f:g:h:i:j:k:l): icmp_seq=1 ttl=51 time=29.1 ms

But the IPv6 address e:f:g:h:i:j:k:l is not actually configured
anywhere on the router (as shown by 'ip a' and other tools)! Is this
really how IP works, that since Verizon is sending packets with that
destination down my connection, my router is going to accept and reply
to them despite not having any such IP address configured?

[1] https://www.verizon.com/support/residential/internet/getting-started/learn-about-ipv6
https://www.dslreports.com/forum/r32136440-Networking-IPv6-working

Celejar