Turn any MAC address into its EUI-64 interface ID and fe80:: link-local address — with every step shown. Add your own /64 prefix for a global address, or reverse an address back to the MAC.
IPv6 interfaces on a LAN autoconfigure a 64-bit interface identifier. The classic method
(modified EUI-64, RFC 4291 Appendix A) derives it deterministically from the interface's
48-bit MAC address in three steps: split the MAC into its 24-bit OUI
(manufacturer) half and 24-bit device half, insert the reserved bytes
ff:fe between them to stretch 48 bits to 64, and flip the U/L bit
— the seventh bit of the first byte — to mark the ID as universally derived. Prefixing the
result with fe80::/64 gives the link-local address every IPv6 interface must
have; prefixing it with a routed /64 gives a global address (SLAAC).
Because the mapping is reversible, EUI-64 addresses leak hardware identity — which is why
modern hosts default to random privacy interface IDs (RFC 4941) for outgoing connections.
Routers, switches, and embedded devices still commonly use EUI-64, so recognizing the
ff:fe signature remains a useful troubleshooting skill.
Use the modified EUI-64 method: split the 48-bit MAC into two 24-bit halves, insert the 16 bits ff:fe between them to get 64 bits, then flip the seventh bit (the universal/local bit) of the first byte. Prepend fe80:: and you have the link-local address. Example: 00:1a:2b:3c:4d:5e becomes fe80::21a:2bff:fe3c:4d5e. This calculator shows each step with the exact bytes and bits.
In MAC addresses the universal/local bit is 0 for globally unique (manufacturer-assigned) addresses. IPv6 inverts the meaning in interface IDs so that manually configured short addresses like fe80::1 have the bit clear, while addresses derived from real burned-in MACs have it set. Flipping bit 7 of the first byte (XOR 0x02) converts between the two conventions.
Only if the interface ID was built with EUI-64: look for ff:fe in the middle of the last 64 bits. Remove those two bytes, flip the U/L bit back, and you have the original MAC — this tool's reverse converter does it for you. Modern operating systems often use random 'privacy' interface IDs (RFC 4941) instead, and those contain no recoverable MAC.
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