Final answer:
The field in an IPv6 Global Unicast Address used for subnetting is the Subnet ID, which allows organizations to create multiple subnets within their network. The Subnet ID typically comprises 16 bits extending the 48-bit Global Routing Prefix. So the correct answer is option D.
Step-by-step explanation:
In an IPv6 Global Unicast Address (GUA), the field used for subnetting is the Subnet ID. The Subnet ID extends the Global Routing Prefix provided by an ISP and is determined by the organization, allowing them to create multiple subnets within their network. An IPv6 GUA is typically composed of the following fields:
- Global Routing Prefix: This is the prefix assigned by the provider and usually comprises the first 48 bits.
- Subnet ID: This field follows the global routing prefix and is used by the organization to define subnets within their network. It can vary in size but typically is 16 bits, resulting in 64-bit network prefix.
- Interface ID: This portion of the address is used to identify individual interfaces within a subnet and is usually 64 bits, following the EUI-64 format.
The process of defining subnets allows networks to be organized internally in a hierarchical manner, improving routing efficiency and management.