About the DUID & IAID Calculator
DHCPv6 identifies clients differently than DHCPv4: instead of the MAC address, clients present a DUID (DHCP Unique Identifier) plus an IAID (Identity Association Identifier) per interface. Building static DHCPv6 reservations means knowing exactly what DUID a client will send — this calculator constructs DUID-LL, DUID-LLT, and IAID values from your inputs.
The format details matter because mismatched reservations fail silently: the client simply gets a dynamic lease instead of the reserved address, and nothing logs why. Computing the expected DUID beforehand turns guess-and-check reservation debugging into a single lookup.
Frequently asked questions
What is the difference between DUID-LL and DUID-LLT?
DUID-LL (type 3) is built from the link-layer (MAC) address alone — stable and predictable, ideal for reservations. DUID-LLT (type 1) adds a timestamp from when the DUID was first generated, making it impossible to predict from the MAC: you must read it off the client. Windows defaults to DUID-LLT, which is why Windows DHCPv6 reservations usually require checking ipconfig /all.
Why doesn't my DHCPv6 reservation match like DHCPv4 did?
DHCPv4 servers key reservations on the MAC; DHCPv6 servers key on DUID (and sometimes IAID). The DUID often is not derivable from the MAC (DUID-LLT, DUID-UUID), is generated per-OS-install rather than per-NIC, and survives NIC replacement. Reimaging a machine typically regenerates its DUID — and quietly breaks its reservation.
What is an IAID and when does it matter?
The IAID distinguishes multiple interfaces on one client — each gets its own identity association under the same DUID. It matters for multi-homed hosts and for servers (like Kea or ISC DHCP) configured to match on DUID+IAID pairs: a reservation tied to the wrong interface's IAID won't apply even though the DUID matches.
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