Địa chỉ Link-Local
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Định nghĩa
Địa chỉ IP chỉ hợp lệ trong một phân đoạn mạng duy nhất, được cấu hình tự động mà không cần DHCP. IPv4 sử dụng 169.254.0.0/16 (APIPA) và IPv6 sử dụng fe80::/10 cho khám phá lân cận và liên lạc cục bộ.
Purpose of Link-Local Addresses
Link-local addresses are valid only on the directly attached network segment — no router will forward them. In IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion., every interface automatically generates a link-local address in the fe80::/10 prefix using Modified EUI-64 from the MAC address, before any stateful or stateless configuration occurs. This means IPv6 link-local addressing is always present, making it essential for neighbor discovery, router solicitation, and other ICMPv6 operations.
IPv4 vs. IPv6 Link-Local
In IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion., the 169.254.0.0/16 range — commonly known as APIPAAutomatic Private IP Addressing. A mechanism where a device self-assigns an IP address from the 169.254.0.0/16 range when no DHCP server is available. APIPA addresses allow local link communication but are not routable beyond the local subnet. — fills the link-local role and is typically assigned only when DHCP fails. IPv6 takes a different approach: fe80:: addresses are mandatory and assigned automatically, enabling the Neighbor Discovery Protocol (NDP) to function even on unconfigured interfaces. Because of this, you will always see an fe80:: address on every active IPv6 interface.
Routing Implications
When configuring static routes or next-hop addresses over point-to-point links, engineers often use the peer's link-local address as the next-hop in OSPFOpen Shortest Path First. An interior gateway routing protocol that uses link-state advertisements and Dijkstra's algorithm to compute the shortest path within an autonomous system., EIGRPEnhanced Interior Gateway Routing Protocol. A Cisco-developed advanced distance-vector routing protocol that uses a composite metric (bandwidth, delay, reliability, load) and supports rapid convergence through its DUAL algorithm., or other routing protocols. Because link-local addresses are unique only per-link, the outgoing interface must also be specified when using them as a next-hop, otherwise the router cannot determine which segment to use. This is a common source of misconfiguration on multi-interface routers.