How is a home/private network's ipv6 network number assigned to Cable Modems?

For typical IPv4 home network deployments, the private network is made up of private IP addresses (usually 192.168.0.0/24). All devices share the router's public (Internet-routable) IP address assigned to it by the ISP via the cable-modem serving as a NAT-router. This serves as both a firewalling protection of the devices on the private network as well as alleviating the need for each device on your home network having a highly-valued IPv4 Internet-valid IP. And for most client devices, this works just fine.

However IPv6 expands the available number of networks and IPs by such a great number that each person on the planet could have more IPs assigned to them than they could possibly use. So NAT-routing isn't really necessary. And to protect the devices, traditional firewalling techniques can be used instead of relying on the devices not being internet-accessible as a form of security.

Now to my point, ISPs issue the cable modem an IPv6 address just like they issue IPv4 addresses. But how do they get assigned the home network IPv6 network number? I understand that IPv6 devices can generate their own host number (lower 64bits of an IPv6 address), but they still need to be told what their network number is (the upper 64-bits). This is normally done via SLAAC or DHCPv6. But the cable modem needs to know what network number to give when devices on the local network ask. 

So, how does the ISP commuincate, to my cable modem, the network number it wants my cable modem issue to my home network IPv6 devices? I'm assuming it needs to issue this so that its routing tables can update to know that return-packets destined to my IPv6 devices must get routed to my cable-modem's IP...unless my Netgear C6300 is NATing IPv6 exactly like it does IPv4.

The IPv6 IPs my cable modem is assigning have a prefix of 2601: (Comcast/Xfinity). When I go to websites that tell me my IP address, they correctly identify both my router's public IPv4 address but they know my device's actual IPv6 address (or at least the IPv6 address being used to communicate with the website) suggesting that the IPv6 IP my device is using is Internet-accessible, and thus isn't being NATed the way my IPv4 communications are.

In traditional commercial router deployments, each router is configured to know the network numbers of its various datalinks. But in the case of residential cable-modems, I'm ASSUMING the cable-modem is told this information. I'm just curious how that's accomplished and if it's a standard networking protocol or something specific to DOCSIS devices. Or perhaps is the cable modem assigned an IPv6 network from the factory and communicates that to the ISP for them to update their routing tables. I can see a number of possibilities and I can't seem to find any answers google-searching. It's evidently not something most people care to know.