pfSense

1/ The mask is absolutely irrelevant for hostname.
2/ You are using the wrong place to do the job, select 'Hosts' from the 'Type' dropdown, not networks.

I am using the right place. I want to create a alias for an ipv6 network not for a ipv6 host. It shoud be possible, it clearly says "Network or FQDN", however the mask changes to /32 as soon as I enter a hostname and that according to the hint should not happen.

And what exactly do you imagine to happen with a thing like www.google.com/128? Yeah, the hint is piece of crap, the netmask is completely irrelevant. Just put the FQDN there and move on. Will get chewed by filterdns and resolved periodically.

You are right /128 is not reasonable, but /64 would be nice.

Example:
www.google.com AAAA record is 2a00:1450:4001:81d::200e

If my understanding is right 2a00:1450:4001:81d::200e/64 would allow the range 2a00:1450:4001:081d:0000:0000:0000:0000-2a00:1450:4001:081d:ffff:ffff:ffff:ffff. And that is what I was going to achieve, since the IPv6 networks I am using are dynamic and can change after a while.

If not whats the point allowing FQDNS in networks, FQDNS should be limited to hosts only.

Well that's an interesting idea but not exactly valid. If 2a00:1450:401b:803::2004 is www.google.com, it doesn't mean that 2@a00:1450:4001:081d::/64@ is www.google.com as well. (If fact it's pretty much never the case.)

If you need something similar, pfBlockerNG package can use ASNs to create aliases via whois.radb.net.

I know that it doesn't mean that it is www.google.com, that wasn't the point we are talking about networks here not one specific host. Maybe I was not clear.

somehostofmyipv6network.com is 2a00:1450:4001:81d::200e
An alias for the whole network would be nice, 2a00:1450:4001:81d::200e/64 in this example. Since the prefix of this Network can change, a FQDN could help. I thought this is what the FQDN option under network was for, but I guess I was wrong.

Thank you anyway for your response.