Hello Martha
When transitioning from IPv4 to IPv6 it’s necessary to reevaluate certain issues. It’s all part of the process! Remember that routing protocols in IPv6 use the link-local addresses to exchange information. The link-local address is automatically assigned to each IPv6 enabled interface and communication between connected IPv6 devices is automatically established using these addresses. For more info on the link-local address, take a look at this lesson:
So since connectivity is established, OSPFv6 messages can be exchanged between the routers. You’ll notice that after IPv6 is enabled on the Ethernet interfaces, in the output of the show ipv6 interface brief command on each router, you see that these routers have an IP address starting with FE80 assigned to them, even though no config was applied.
As for the subnet mask or the prefix of the loopback addresses, the prefix used here is analogous to using the /32 prefix in IPv4. So you can have one loopback be assigned 192.168.55.5/32 and the loopback on the other router be assigned 192.168.55.6/32. The /128 mask simply states that the IP address belongs to a range that contains only one address: itself.
I hope this has been helpful!
Laz