LinkLocal

In a computer network, a link-local address is a network address that is valid only for communications within the network segment (link) or the broadcast domain that the host is connected to.

Link-local addresses are not guaranteed to be unique beyond a single network segment. Routers therefore do not forward packets with link-local addresses.

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 the address block fe80::/64.

Link-local addresses may be assigned manually by an administrator or by automatic operating system procedures. For (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 (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 169.254.0.1 to 169.254.255.254 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.

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