Leaving aside the addressing ‘theory’, in reality the IPv6 address space has been partitioned into a few areas which have a fixed meaning. You can see the allocation in Table 3-1.

The vast majority of the address space is occupied by global unicast addresses. But just a tiny part of this allocation is really used – no more than three /16 prefixes (see Table 3-2, “Global unicast prefixes in real use”).

The 2002::/16 prefix is dedicated to 6to4 mechanism allowing automatic tunnelling of IPv6 datagrams over common IPv4 Internet (described later in this book). The 3ffe::/16 prefix was used in the 6bone network – an experimental overlay network intended to gain practical experience with IPv6 operation. Allocations from this address space have been stopped. The support (routing) of these addresses will be deprecated at the end of June 2006 (RFC 3701 describes the 6bone phase-out schedule). In consequence the usage of 3ffe::/16 prefix drops and many 6bone addresses have already been abandoned.

The most important address prefix is the 2001::/16 from which all the regular global unicast addresses in the contemporary Internet originate. Allocation of addresses from this prefix is managed by Regional Internet Registries (RIRs) – the same organisations which also manage the IPv4 address space.

Every RIR has obtained some part of the 2001::/16 prefix from which it allocates smaller parts. Regions covered by a single RIR are quite large (e.g. a continent), so the allocation is not made directly. It is made in collaboration with Local Internet Registries (LIRs), which are typically the Internet providers. The mechanism is similar – every LIR obtains some part of the address space from which it allocates prefixes to its customers.

All the RIRs have agreed on common allocation rules and the address structure displayed in Figure 3-5. This clear and well-arranged structure is the result of a few years of evolution. One of the really nice features of this contemporary addressing structure is that the borders between different authorities are situated at the colons in the address written form.

The first four hexadecimal digits are fixed, they are 2001. The next four are assigned by the RIRs. It means that if some LIR asks for an address space to manage it obtains a 32-bit prefix from which 48- bit prefixes are allocated to its customers (end-sites). So the specification of the third quadruple is up to the LIR. Finally, the last foursome of hexadecimal digits in the first half of the address contains the subnet ID. This is assigned by the local network manager.