IPv6 prefix delegation

Hello,

Sorry if this question has been asked before, I couldn't find the answer I was looking for in the forum.

I have my own modem and firewall set up. Right now I am using ipv4 and all is well but I would like to use IPv6 and I'm not sure how to get it working through the FW.

I have a Cisco ASA 5506-X. I know I need to enable IPv6 globally and I know I need to add prefix delegation. How do I know what the delegation is going to be? Does Comcast/Xfinity supply that? Also, if I have a ::/56 - does that mean I have all of the associated networks that I can utilize?

TIA -

Nademan