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An autoconfiguration IP address is an address a device assigns to itself when it cannot use another IPv4 configuration. In the common IPv4 case, it is an address in 169.254.0.0/16: it can support communication with devices on the same network link, but routers do not forward it, so it does not provide ordinary internet access.
What does “autoconfiguration IP address” mean?
The phrase usually means an IPv4 link-local address configured automatically under the rules in RFC 3927, Dynamic Configuration of IPv4 Link-Local Addresses. The address comes from 169.254.0.0/16 and is intended for communication between devices on the same physical or logical link. It is often called an APIPA address, but APIPA is a common implementation term, not the name of the IETF standard.
A device may choose this fallback when another usable IPv4 configuration is unavailable, for example when it cannot find a DHCP server. The address tells you what configuration the device has, not exactly why it ended up with it.
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What can a 169.254 address reach?
Devices on the same link may communicate using compatible link-local configuration. However, routers must not forward packets with a source or destination in 169.254.0.0/16, as specified by RFC 3927 and the address-block guidance in RFC 5735. That means a 169.254 address is not ordinary routed network or internet connectivity. It is also not accurate to say the device has no network communication at all: local-link communication can still work.
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How does IPv4 link-local address selection work?
The dynamic selection range is 169.254.1.0 through 169.254.254.255. The first 256 and last 256 addresses in the larger block are reserved and excluded from this selection mechanism. A host chooses an address pseudorandomly, probes to check that it is not already in use, and must continue to detect and handle conflicts while using it. This matters if networks that were previously separate are later connected. These requirements are defined in RFC 3927.
RFC 3927’s 2005 analysis estimates a 98% chance of choosing an unused address on the first try and a 99.96% chance within two tries. Those are estimates based on the standard’s selection analysis, not current measurements of devices in the field.
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Is IPv6 link-local addressing the same thing?
No. IPv6 has a separate link-local and autoconfiguration mechanism. RFC 4862 describes IPv6 Stateless Address Autoconfiguration and says every IPv6 interface has a link-local unicast address, whose scope is the same link. Unlike the IPv4 fallback case, an IPv6 link-local address is a normal interface feature and does not, by itself, indicate that DHCP failed. The IPv4 block 169.254.0.0/16 does not apply to IPv6.
What should you check if your device has a 169.254 address?
The address alone cannot identify the precise local cause. A disconnected cable, Wi-Fi association issue, DHCP service problem, or another configuration condition may be involved; the standards define the address behavior, not a universal operating-system diagnosis.
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- Check the link. Confirm the cable is connected or that the device is associated with the intended Wi-Fi network.
- Check the intended IPv4 configuration. If the network should assign addresses automatically, confirm its DHCP service and the device’s network configuration are working.
- Restore a routable configuration if you need off-link access. A usable routable IPv4 address is needed for ordinary communication beyond the local link. RFC 3927 says that when such an address becomes available, a host must use it for new communications and stop advertising its link-local address as available, apart from transition behavior.
Should you assign 169.254 addresses manually?
No. RFC 3927 says the 169.254/16 block should not be configured manually or used for a DHCP pool. For administrator-assigned local IPv4 addresses, use an appropriate private range from RFC 1918 instead. A link-local address is designed for automatic, same-link use, not as a substitute for planned LAN addressing.
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