Automatic Private IP Addressing (APIPA)

Automatic Private IP Addressing (APIPA) is a vendor name for IPv4 link-local addressing, which RFC 3927 standardises. When a host is set to obtain its address automatically and no DHCP server responds, it picks an address at random from the 169.254.0.0/16 block, uses ARP to check that no neighbour already holds it, and assigns it to itself. Other operating systems do the same thing under the plain name link-local addressing. IPv6 has its own link-local range, fe80::/10, which every IPv6 interface carries whether or not it has other addresses.

A link-local address works only on the local segment. Routers do not forward packets to or from it, so the host can reach neighbours that also hold 169.254 addresses but not other subnets or the internet. These addresses are neither private addresses in the RFC 1918 sense nor public ones, and the difference helps when reading a fault: an RFC 1918 address commonly means the host received a configuration, while a 169.254 address means it configured itself because nothing answered.

Exam relevance: a scenario is likely to describe a user who cannot reach network resources and whose machine shows a 169.254 address. The expected diagnosis is that the host could not reach a DHCP server. Candidates are expected to recognise the range and what it signals.