The first few bits of an IPv6 address which tell a host what kind of address it is.
0000 0000 - Used for things like IPv6 compatible IPv4 addresses (1/256th of the address space) 001 - Aggregatable Global Unicast Addresses (1/8 of the address space) 1111 1110 10 - Link-Local Unicast Addresses (1/2024 of the address space) 1111 1110 11 - Site-Local Unicast Addresses (1/2024 of the address space) 1111 1111 - Multicast Addresses (1/256 of the address space)
Almost all the other address space is unallocated or reserved.
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