RFC 3879 [6], and partially replaced by Unique Local Address (ULA) [7]. The ULA addresses can be distinguished by specific prefix of fc00 : 17, as illustrated in Figure 3.3. At the time of writing, RFC 4193 specifies how ULAs are generated for local assignment, and hence the 'L'-bit is set to '1' (and hence making the value of the first 8 bits fd). The locally allocated ULA prefixes use a pseudo random global identifier (global ID). The subnet identifier (subnet ID) is then assigned administratively in a network as required. IPv6 implementations need to be able to differentiate ULA addresses from other global scope addresses for address selection purposes, as is described in Section 3.5.4. For scopes of multicast addresses, please see Section 3.2.4. 3.1.4 IPv6 Addressing Zones As stated in RFC 4007 [5]: 'a zone is a connected region of topology of given scope'. For example, for globally scoped addresses there is one zone, Internet, while for link- local scoped addresses there are as many zones as there are links. If an application is communicating to a non-global zone, it may need to indicate the zone identifier to lower layers, as the zone is not determinable from the IPv6 address itself - for example, all link- local addresses start with fe80: : and that gives no hint about what link, or interface, it belongs to. Please see Section 3.1.9 about textual presentations of zones. The zone identifiers are not typically communicated between peers, as the identifiers are implementation specific and generally have no meaning for other nodes than the one that has assigned an identifier to a zone. 3.1.5 IPv6 Addresses on Network Interfaces In IPv6 every address is assigned to an interface. Each interface also usually has a multitude of addresses, of the same and different scopes. In addition to addresses directly allocated to interfaces, nodes are listening to a set of multicast and, possibly, anycast addresses. The exact addresses that nodes listen to depend on the role of the node. For example, every node listens to All