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, loopback and unspecified 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.