IPv6 Address

An IPv6 address is a 128-bit identifier for a network interface, written as eight groups of four hexadecimal digits separated by colons, for example 2001:0db8:85a3:0000:0000:8a2e:0370:7334. Two compression rules make the notation manageable: leading zeros inside each hextet may be dropped, and a single run of all-zero hextets may be replaced with ::. The :: shortcut can appear only once per address so the expanded form remains unambiguous.

The 128-bit space (~3.4 × 10³⁸ addresses) removes the scarcity that drives IPv4 NAT deployments. An IPv6 prefix identifies the network portion, and on most user-facing links the host portion is exactly 64 bits — a constraint that SLAAC depends on. Addresses are categorised by scope: global unicast (2000::/3) routes on the public Internet, link-local (fe80::/10) is valid only on a single link, unique local (fc00::/7) is the IPv6 analogue of private IPv4, and multicast lives in ff00::/8. There is no broadcast; its role is taken by well-known multicast groups such as ff02::1.

Global allocation flows from IANA to the RIRs to LIRs and finally end sites, typically as a /32 to an RIR member, /48 to a site, and /56 to a residential customer. Because aggregation is enforced at every step, the global IPv6 routing table is far smaller than the IPv4 table for the same number of networks. Renumbering, however, becomes a real operational concern: an address hard-coded somewhere keeps working only as long as its prefix is announced.