What is the typical size of the interface identifier in an IPv6 global unicast address?

In an IPv6 global unicast address, the interface identifier is typically 64 bits long. This is a standardized size that allows for a very large number of unique addresses for individual interfaces on the internet.

The structure of an IPv6 address is divided into two main parts: the prefix and the interface identifier. The first 64 bits are used for the network prefix, which identifies the specific network to which the address belongs, while the last 64 bits serve as the interface identifier, which uniquely identifies a host or interface on that network.

Using a 64-bit interface identifier not only supports a vast address space but also aligns with the requirement for stateless address autoconfiguration (SLAAC) protocols, where the identifier can be generated from the MAC address of the interface.

In contrast, other sizes mentioned in the options do not adhere to the standards for IPv6 addressing. For example, 32 bits would be too short to accommodate the necessary uniqueness for hosts in a global scenario, while 48 bits is sometimes used in MAC addresses but does not fit within the IPv6 addressing structure. Lastly, 128 bits is the total size of an IPv6 address, not just the interface identifier.