APIPA
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Định nghĩa
Tự động Gán Địa chỉ IP Riêng (Automatic Private IP Addressing). Cơ chế mà thiết bị tự gán địa chỉ IP từ dải 169.254.0.0/16 khi không có máy chủ DHCP khả dụng. Địa chỉ APIPA cho phép liên lạc trong liên kết cục bộ nhưng không thể định tuyến ra ngoài mạng con cục bộ.
How APIPA Works
When a host running Windows or another APIPA-capable OS fails to receive a DHCPDynamic Host Configuration Protocol. A network protocol that automatically assigns IP addresses, subnet masks, gateways, and DNS servers to devices when they join a network. lease — because the server is unreachable or the network has no DHCP server — it automatically self-assigns an address in the 169.254.0.0/16 range. The host first sends ARP probes to verify the chosen address is not already in use on the segment, then configures the interface. This process is also defined in RFC 3927 as IPv4 Link-Local.
Scope and Limitations
APIPA addresses are strictly Link-Local AddressAn IP address valid only within a single network segment, automatically configured without DHCP. IPv4 uses 169.254.0.0/16 (APIPA) and IPv6 uses fe80::/10 for neighbor discovery and local communication.: routers will never forward packets sourced from or destined for the 169.254.0.0/16 range. This means APIPA-configured hosts can communicate only with other APIPA hosts on the same Layer 2 segment — they cannot reach the internet, DNSDomain Name System. The hierarchical, distributed naming system that translates human-readable domain names (e.g., example.com) into IP addresses (e.g., 93.184.216.34). Often called the "phonebook of the internet." servers, or any device on a different subnetA 32-bit number (e.g., 255.255.255.0) that divides an IP address into network and host portions. It determines which part of the address identifies the network and which part identifies individual devices.. Spotting 169.254.x.x addresses in a network scan is a reliable indicator that DHCP acquisition failed on those hosts.
Diagnostic Value
Network engineers treat APIPA as an early-warning signal. If a large number of devices in a VLAN self-assign 169.254.x.x addresses simultaneously, the DHCP server has likely crashed, the DHCP scope has been exhausted, or a rogue device is rejecting OFFER packets. Checking for APIPA is a standard first step in LAN troubleshooting before escalating to DHCP server logs. Subnet Calculator