How to Calculate Subnet Mask from IP Address

Since there are 128 bits in an IPv6 network address, it is easy. IPv6 addresses, with a few exceptions, have a 64-bit network portion and a 64-bit interface (host) portion. Basically, you are taking the first half (high-order bits) of the address for the network portion, and the second half (low-order bits) of the address for the interface portion.

The general format for IPv6 Global Unicast addresses is as follows:

|         n bits         |   m bits  |       128-n-m bits         |
+------------------------+-----------+----------------------------+
| global routing prefix  | subnet ID |       interface ID         |
+------------------------+-----------+----------------------------+

RFC 4291, IP Version 6 Addressing Architecture will give you the basics, and RFC 5952, A Recommendation for IPv6 Address Text Representation will give you the canonical representation.

FYI, the IPv6 address 0000:0000:0000:0000:0000:0000:0000:0000 is canonically represented as :: and is called the Unspecified address. Also, the address you list, FC00::50 is in the part of the Unique Local address range that is currently undefined; FD00::50 would be in the part of the Unique Local address range that is defined, but it would be missing the required Global ID.

With a very few exceptions, IPv6 will use /64 networks: 64 bits for the Network ID, and 64 bits for the Interface ID. You may see /128 for a host address, and /127 or /126 for a point-to-point link. When you see anything less than /64, e.g. /48, you are seeing a network block that is to be divided into /64 networks.

IPv6 notation only uses the CIDR notation, not a network mask. IPv4 addresses are 32 bits, so the maximum mask length is /32. IPv6 addresses are 128 bits, so the maximum mask length is /128.

You should investigate RFC 4291, IP Version 6 Addressing Architecture, and RFC 5952, A Recommendation for IPv6 Address Text Representation.