Link-local address

In a computer network, a link-local address is a network address that is valid only for communications within the network segment or the broadcast domain that the host is connected to. Link-local addresses are most often assigned automatically through a process known as stateless address autoconfiguration or link-local address autoconfiguration. In a computer network, a link-local address is a network address that is valid only for communications within the network segment or the broadcast domain that the host is connected to. Link-local addresses are most often assigned automatically through a process known as stateless address autoconfiguration or link-local address autoconfiguration. Link-local addresses are not guaranteed to be unique beyond their network segment, therefore routers do not forward packets with link-local addresses. 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::/10. Link-local addresses may be assigned manually by an administrator or by automatic operating system procedures; this process is also known as self-assigned IP addressing. 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 (IETF) has reserved the IPv4 address block 169.254.0.0/16 (169.254.0.0 – 169.254.255.255) for link-local addressing. The entire range may be used for this purpose, except for the first and last 256 addresses (169.254.0.0/24 and 169.254.255.0/24), which are reserved for future use and must not be selected by a host using this dynamic configuration mechanism. 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.