Final answer:
The standard prefix length for an IPv6 subnet is /64, leaving 64 bits for the host portion. If an ISP issues a different prefix length, subtract this from 128 to find the number of bits available for subnetting.
Step-by-step explanation:
The question pertains to the calculation of usable subnets within a given Global Unicast Address provided by an ISP (Internet Service Provider) for a company. In the context of IPv6, the standard prefix length for a subnet is typically /64, meaning that the first 64 bits of the address are used for the network portion, including the global routing prefix and the subnet ID. The remaining 64 bits are used for interface identifiers, which is the host portion. If the ISP has issued a prefix with a different length, the number of bits available for subnetting will be 128 minus the length of the prefix given by the ISP. For example, if the ISP issued a /48 prefix, the company could use the next 16 bits for subnetting, creating up to 2^16 possible subnets.