I have been given an IPv6 address of
2a00:da10:f218:9ff0::1
And I read that ::1 is the local loopback.
But the public display I've been given by the provider shows:
2a00:da10:f218:9ff0::/80
But this is a syntax error when I use it on telnet etc. I have also tried telnet with
2a00:da10:f218:9ff0::
but this also fails.
What is the correct syntax to implement this address in places like DNS etc.
And I read that ::1 is the local loopback.
Well, yes, but only precisely ::1 is local loopback – that's not an abbreviation, that's actually the whole address (which is 0:0:0:0:0:0:0:1 in full).
It doesn't apply to other addresses that end with "[…]::1", so your 2a00:da10:f218:9ff0::1 is not ::1 and therefore not loopback – it's a normal global address.
But the public display I've been given by the provider shows: 2a00:da10:f218:9ff0::/80
It shows you a network – an address range, not a single address. The /80 has the same meaning as it does in IPv4; it is a short way of writing the subnet mask and indicates the network size (the range start-end can be determined from it).
Your /80 is therefore a range from 2a00:da10:f218:9ff0:: (aka 2a00:da10:f218:9ff0:0:0:0:0 if you want) to 2a00:da10:f218:9ff0:0:ffff:ffff:ffff.
For comparison, the IPv4 equivalent for your home LAN would be 192.168.1.0/24.
What is the correct syntax to implement this address in places like DNS etc
The 2a00:da10:f218:9ff0::1 address that you were given is the one that you should use in DNS.
Unless, of course, you configure the server to have additional addresses from the range you've been given – then you could use those in DNS as well.
(The entire /80 is available for your use, though the all-zeros address should be avoided as it coincides with the usually-reserved 'subnet routers anycast' special address.)
