For protocols that have only link-local addresses, such as Ethernet, hardware addresses assigned by manufacturers in networking elements are unique, consisting of a vendor identification and a serial identifier.
Link-local addresses for IPv4 are defined in the address block 169.254.0.0/16, in CIDR notation. In IPv6, they are assigned with the prefix fe80::/64, although being defined as the block fe80::/10.
Link-local addresses may be assigned manually by an administrator or by automatic operating system procedures. For Internet Protocol (IP) networks, they are assigned most often using stateless address autoconfiguration. In IPv4, they are normally only used to assign IP addresses to network interfaces when no external, stateful mechanism of address configuration exists, such as the Dynamic Host Configuration Protocol (DHCP), or when another primary configuration method has failed. In IPv6, link-local addresses are mandatory and required for the internal functioning of various protocol components.
Automatic address configuration of link-local addresses is often non-deterministic as the resulting address cannot be predicted. However, in IPv6 it is usually derived automatically from the interface media access control (MAC) address in a rule-based method.
In RFC 3927, the Internet Engineering Task Force has reserved the address block 169.254.0.0/16. From this block, the address range 169.254.1.0 to 169.254.254.255 may be used for link-local addressing in Internet Protocol Version 4. Link-local addresses are assigned to interfaces by host-internal, i.e. stateless, address autoconfiguration when other means of address assignment are not available.
RFC 3927 warns against the simultaneous use of IPv4 addresses of different scope, such as configuring link-local addresses as well as globally routable addresses on the same interface. Therefore, hosts search for a DHCP server on the network before assigning link-local addresses.
In the automatic address configuration process, network hosts select a random candidate address within the reserved range and use Address Resolution Protocol (ARP) probes to ascertain that the address is not in use on the network. If a reply is received to the ARP, it indicates the candidate IP address is already in use; a new random candidate IP address is then created and the process repeated. The process ends when there is no reply to the ARP, indicating the candidate IP address is available.
When a globally routable or a private address becomes available after a link-local address has been assigned, the use of the new address should generally be preferred to the link-local address for new connections but communication via the link-local address is still possible.
In the Internet Protocol Version 6 (IPv6), the address block fe80::/10 has been reserved for link-local unicast addressing. To conform to standard /64 addressing on subnets, the actual link local addresses are assigned with the prefix fe80::/64. The 54 bits after the most significant ten bits must be zero.
Unlike IPv4, IPv6 requires a link-local address on every network interface on which the IPv6 protocol is enabled, even when routable addresses are also assigned. Consequently, IPv6 hosts usually have more than one IPv6 address assigned to each of their IPv6-enabled network interfaces. The link-local address is required for IPv6 sublayer operations of the Neighbor Discovery Protocol, as well as for some other IPv6-based protocols, such as DHCPv6.
When using an IPv6 link-local address to connect to a host, a zone index must be added to the address so that the packets can be sent out on the correct interface.
In IPv6, addresses may be assigned by automatic (stateless) or stateful (e.g. manual) mechanisms. Stateless address autoconfiguration is performed as a component of the Neighbor Discovery Protocol (NDP), as specified in RFC 4862. The address is formed from its routing prefix and the MAC address of the interface.
Through NDP routing prefix advertisements, a router or server host may announce configuration information to all link-attached interfaces which causes additional IP address assignment on the receiving interfaces for local or global routing purposes. This process is sometimes also considered stateless, as the prefix server does not receive or log any individual assignments to hosts. Uniqueness is guaranteed automatically by the address selection methodology. It may be MAC-address based according to RFC 4862, or randomized according to RFC 4941. Automatic duplicate address detection algorithms prevent assignment errors.
Media access control
Media access control (MAC) addresses used in local broadcast networks, such as Ethernet, are link-local addresses. Such devices are configured with an address in hardware by the manufacturer. However, operating system utilities, such as ifconfig may be used to assign or change these addresses. This is essential when an operating system is running in a virtual machine environment.
- RFC 3927, Dynamic Configuration of IPv4 Link-Local Addresses, S. Cheshire, B. Aboba, E. Guttman, The Internet Society (May 2005)
- RFC 4291,IP Version 6 Addressing Architecture, R. Hinden, S. Deering, The Internet Society (February 2006)
- RFC 3927 section 1.9
- RFC 3927 section 2.6.1
- "APIPA". Microsoft. Retrieved 2010-08-02.
- RFC 4291, section 2.5.6. Link-Local IPv6 Unicast Addresses
- RFC 4291, section 2.8. A Node's Required Addresses
- RFC 4862, IPv6 Stateless Address Autoconfiguration, S. Thompson, T. Narten, T. Jinmei (September 2007)