Hosts on a network must be assigned [[IP address]]es that uniquely identify them to other devices on the same network. On some networks there is a central authority that assigns these addresses as new devices are added. Mechanisms were introduced to handle this task automatically, and both IPv4 and IPv6 now include systems for [[address autoconfiguration]], which allows a device to determine a safe address to use through simple mechanisms. For [[link-local address]]ing, IPv4 uses the special block {{IPaddr|169.254.0.0|16}},{{Ref RFC|3927}} while IPv6 hosts use the prefix {{IPaddr|fe80::|10}}. More commonly addresses are assigned by a [[DHCP server]], often built into common networking hardware like computer hosts or routers.
Most IPv4 hosts use link-local addressing only as a last resort when a DHCP server is unavailable. An IPv4 host otherwise uses its DHCP-assigned address for all communications, global or link-local. One reason is that IPv4 hosts are not required to support multiple addresses per interface, although many do. Another is that not every IPv4 host implements distributed name resolution (e.g., [[multicast DNS]]), so discovering the autoconfigured link-local address of another host on the network can be difficult. Discovering the DHCP-assigned address of another host requires either distributed name resolution or a unicast DNS server with this information; Some networks feature DNS servers that are automatically updated with DHCP-assigned host and address information.
IPv6 hosts are required to support multiple addresses per interface; moreover, every IPv6 host is required to configure a link-local address even when global addresses are available. IPv6 hosts may additionally self-configure additional addresses on receipt of router advertisement messages, thus eliminating the need for a DHCP server.{{Ref RFC|4862}}
Both IPv4 and IPv6 hosts may randomly generate the host-specific part of an autoconfigured address. IPv6 hosts generally combine a prefix of up to 64 bits with a 64-bit EUI-64 derived from the factory-assigned 48-bit [[IEEE]] [[MAC address]]. The MAC address has the advantage of being globally unique, a basic property of the EUI-64. The IPv6 protocol stack also includes duplicate address detection to avoid conflicts with other hosts. In IPv4, the method is called ''link-local address autoconfiguration''.{{Ref RFC|3927}} However, [[Microsoft]] refers to this as ''Automatic Private IP Addressing'' ([[APIPA]])<ref>{{Citation | url = http://msdn.microsoft.com/en-us/library/aa505918.aspx | publisher = Microsoft | title = MS Developer Network | contribution = Apipa | access-date = 2008-07-05 | archive-url = https://web.archive.org/web/20170318001826/https://msdn.microsoft.com/en-us/library/aa505918.aspx | archive-date = 2017-03-18 | url-status = dead }}</ref> or '''''Internet Protocol Automatic Configuration''''' ('''IPAC'''). The feature is supported in Windows since at least [[Windows 98]].<ref>{{Citation | url = http://support.microsoft.com/kb/220874 | title = Knowledge base | contribution = How to use automatic TCP/IP addressing without a DHCP server | publisher = Microsoft}}</ref>
