Those of you who are familiar with IPv4 networks may have encountered the notion of private versus public address space. Private address space is address space used within an organization's network, and in theory it cannot be reached from the outside world (often people like to pretend that this gives them additional security, see "NAT" in Chapter 1). These addresses are an example of address spaces with special properties—and often (but not always) these types of address space can be inferred by glancing at the address.
Examples of special addresses from the IPv4 world include the
private class A space
which is discussed in the "Addressing Model" section in Chapter 1, and would be familiar to
those building enterprise networks. Similarly
127.0.0.0/8 is the "localhost" space, which
hosts use to contact themselves.
One interesting IPv4 special address is the "broadcast" address in IPv4, 255.255.255.255, because it has no direct equivalent in IPv6. Broadcasts no longer exist in IPv6, and multicast is used as the transport for contacting multiple hosts simultaneously.
Similarly in IPv6 there are a number of address spaces, usually expressed as a prefix with CIDR network length. The official breakup of this space is documented on the IANA web site http://www.iana.org/, but we summarize the allocations in Table 3-1.
Table 3-1. The breakup of the IPv6 address space