What is modified EUI-64 in IPv6?

Modified EUI-64 is a 64-bit interface identifier built from a 48-bit MAC address: FF:FE is inserted in the middle, and the universal/local (U/L) bit in the first octet is inverted. That value becomes the host half of many IPv6 addresses.

How do I convert a MAC address to an IPv6 link-local address?

Derive the modified EUI-64 interface ID from the MAC, then prepend the link-local prefix fe80::/64. For example, MAC 00:1B:44:11:3A:B7 becomes fe80::21b:44ff:fe11:3ab7.

Why do we flip the seventh bit (U/L bit)?

IEEE EUI identifiers use the U/L bit to mark universal vs locally administered addresses. IPv6 inverts that bit when forming a modified EUI-64 interface ID so manually configured local IDs can stay simple (for example ::1) while still following the rules in RFC 4291.

Can I convert IPv6 back to a MAC address with this tool?

Yes, when the interface ID still follows classic modified EUI-64 with FF:FE in the middle. Paste the IPv6 address and the calculator extracts the interface ID and reverses the U/L bit flip to recover the MAC. Random privacy addresses will not reverse cleanly.

Do modern operating systems still use EUI-64 for SLAAC?

Many hosts now use temporary or stable privacy addresses instead of exposing the MAC through EUI-64. You will still see EUI-64 in older devices, some embedded systems, documentation, and certification exams such as CCNA.

Does this IPv6 to EUI-64 calculator send my addresses to a server?

No. All conversion steps run locally in your browser. We do not log MAC addresses or IPv6 addresses you paste into the tool.

IPv6 to EUI-64 Calculator, MAC to Modified EUI-64 & Link-Local

IPv6 to EUI-64 Calculator, MAC to Modified EUI-64 & Link-Local | Binary Tech