Final answer:
The ISP assigns the Global Routing Prefix part of a GUA. This prefix is crucial for the Internet's hierarchical addressing structure and routing. Although the ISP does not assign the Subnet ID, it is influenced by the initial Global Routing Prefix allocation. The correct answer is A. Global routing prefix.
Step-by-step explanation:
The part of a Global Unicast Address (GUA) that is assigned by an Internet Service Provider (ISP) is the Global Routing Prefix. This prefix is a key component in the hierarchical addressing architecture of the Internet. It typically identifies the network segment that is part of the ISP's allocated address range and is used to facilitate routing on the Internet.
The size of the global routing prefix can vary, but for IPv6, a common size is a /48 or /56 prefix, leaving adequate space for a large number of subnets within a customer's network.
Subnet IDs are also determined by the organization or the end-user to further segment the network internally, allowing efficient traffic management and security within their own networks but are not assigned by the ISP.
These assignments follow the ISPs initial allocation of the global routing prefix and allow organizational control over their internal network structure. IPv6 addressing provides a vast amount of address space, enabling this kind of detailed subnetting without running into the scarcity issues that were common with IPv4.