EUI-64 Interface ID Calculator
Turn a 48-bit MAC address into its Modified EUI-64 IPv6 interface identifier — splitting the MAC, inserting FFFE, and flipping the U/L bit. Add an optional /64 prefix to build the complete IPv6 address.
001a.2b3c.4d5e) or bare hex.
fe80:: for the link-local address, or enter your own network prefix.
When an IPv6 interface configures itself, one classic way to fill in the lower 64 bits — the interface identifier — is to derive them from the network card's 48-bit MAC address using the Modified EUI-64 procedure. This calculator performs that transformation and, optionally, glues the result onto a /64 prefix to show the full address.
The algorithm has three steps. Split the MAC into two 24-bit halves; insert the fixed 16-bit value FFFE between them to stretch 48 bits into 64; then flip the seventh bit of the first octet — the universal/local (U/L) bit. That final flip is the detail people forget: a MAC beginning 00 becomes 02, and 00:1a:2b:3c:4d:5e turns into the interface ID 021a:2bff:fe3c:4d5e.
Because an EUI-64 interface ID embeds the hardware MAC, it can track a device across networks. Modern operating systems therefore default to randomised privacy addresses (RFC 8981) instead. EUI-64 is still widely used on routers and link-local addresses, and understanding it makes IPv6 autoconfiguration far less mysterious.
Why flip the U/L bit at all? In a MAC address a 0 in that position means "globally unique"; IPv6 inverts the convention so a 1 means global scope, which keeps hand-written interface IDs short. Flipping the bit simply translates between the two worlds. To dig into the source of these addresses, read what a MAC address is, and for the broader protocol picture see IPv4 vs IPv6. You can reformat the input MAC with the MAC Address Formatter, or take the resulting address into the IPv6 Expand & Compress tool. Nothing you enter is uploaded — it all runs locally.
Frequently asked questions
How is a Modified EUI-64 interface ID built from a MAC address?
Split the 48-bit MAC in half, insert the 16 bits FFFE in the middle, then flip the 7th bit (the universal/local bit) of the first octet. For 00:1a:2b:3c:4d:5e that gives the interface identifier 021a:2bff:fe3c:4d5e.
Why is the U/L bit flipped?
In a MAC address, a 0 in the universal/local bit means the address is globally unique. IPv6 inverts the meaning so that a 1 indicates global scope, which makes manually configured interface IDs shorter and simpler. Flipping the bit converts between the two conventions, so the first octet 00 becomes 02.
Is EUI-64 still used for IPv6 addresses?
It is less common than it used to be. Because an EUI-64 address exposes the device MAC, most operating systems now default to randomised privacy addresses (RFC 8981). EUI-64 is still used on many routers and point-to-point links, and it remains essential to understand link-local addressing.
Can I build a full IPv6 address from the interface ID?
Yes. Enter a /64 prefix such as fe80:: or 2001:db8:abcd:1::, and this tool combines it with the EUI-64 interface identifier to produce the complete 128-bit address, shown in both compressed and expanded form.
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