What is the size of the 'global ID' in an IPv6 unique local address?

In an IPv6 unique local address, the 'global ID' is indeed 40 bits in size. Unique local addresses (ULAs) are similar to private addresses in IPv4, allowing for local communication within a site or organization without needing a globally unique address. The structure of a unique local address is defined as follows:

• The first 7 bits are always set to 1111110, which identifies it as a unique local address.

• The next 40 bits constitute the global ID, which is typically assigned randomly or generated in such a way to ensure uniqueness within the local context.

• Following the global ID, there is a 16-bit subnet ID, and finally, a 64-bit interface ID that identifies a specific interface on the host.

This structure highlights the significance of the 40-bit global ID, as it allows for a considerable number of unique local address ranges, supporting organizational requirements without overlaps in private addressing.