linklocal

Link-local refers to a class of network addresses that are valid only on a single network segment or link. These addresses are not intended to be routable across different networks and are used for local communication and initial network operations such as auto-configuration and neighbor discovery.

In IPv4, link-local addresses occupy the 169.254.0.0/16 range. They are assigned automatically by a device if it cannot obtain an address from a DHCP server, a mechanism commonly known as Automatic Private IP Addressing (APIPA). Before using a link-local IPv4 address, a device performs duplicate address detection (usually via ARP) to ensure the address is not already in use. Because they are not routable, these addresses are meant only for communications on the local link and are typically used during small networks, DHCP failures, or direct device-to-device connections.

In IPv6, link-local addresses use the FE80::/10 prefix and are generated on every IPv6-enabled interface. A link-local address is required for IPv6 operation and is used for essential link-local functions such as neighbor discovery, router discovery, and stateless address autoconfiguration. Link-local IPv6 addresses are always scoped to the local link and packets with such addresses are not routed beyond the local network segment. Routers do not forward link-local traffic.

Both IPv4 and IPv6 link-local addressing provide a fail-safe mechanism for local network operation, enabling devices to communicate and perform basic network tasks even in the absence of external configuration or routing infrastructure.