Why is the size of the interface identifier set to 64 bits in IPv6 addresses?

The interface identifier in IPv6 addresses is set to 64 bits primarily to ensure a vast number of possible host addresses within the network. This length allows each subnet to support a significantly larger number of devices, which is crucial given the expectations for the growth of the Internet and the increasing number of connected devices.

By using 64 bits for the interface identifier, IPv6 can allow for approximately 18 quintillion unique addresses per subnet, vastly exceeding the limitations posed by IPv4 addressing. This level of address space is essential to accommodate future growth and the diverse range of devices expected to connect to the internet, such as smartphones, IoT devices, and other connected systems.

The other options, while relevant to networking principles, do not convey the core reason for the specification of the 64-bit size of the interface identifier in the IPv6 addressing scheme. Conformity to standards, simplified configurations, and matching IPv4 are not the primary motivations for this decision. The emphasis is instead on providing ample addressing capacity to support a growing network environment.