How are IPv6 Link-Local Addresses (LLA) dynamically created?

An interface auto-builds its LLA by combining the fe80::/10 prefix with an Interface ID generated by EUI-64 or a random 64-bit number.

Dynamic LLA Creation:

All IPv6 interfaces must have an LLA. It can be:

• Dynamically created (automatic)
• Statically configured (manual)

Dynamic LLA Structure:

fe80::/10 prefix + Interface ID (EUI-64 or random)

EUI-64 Generated LLA:

MAC: fc:99:47:75:ce:e0
LLA: fe80::fe99:47ff:fe75:cee0

Random 64-bit Generated LLA:

LLA: fe80::50a5:8a35:a5bb:66e1 (random)

Windows ipconfig output shows both:

IPv6 Address. . . . : 2001:db8:acad:1:50a5:8a35:a5bb:66e1
Link-local IPv6 Address: fe80::50a5:8a35:a5bb:66e1
Default Gateway . . : fe80::1

Key insight: The Interface ID portion is often the same for both GUA and LLA when using the same generation method.

Go deeper:

• RFC 4862 §5.3 — Creation of Link-Local Addresses — how an interface forms its LLA from fe80::/10 plus an interface identifier.
• Link-local address — IPv6 (Wikipedia) — fe80::/10 auto-configuration with EUI-64 or random interface IDs.