IPv6 EUI-64 Utility

IPv6 EUI-64 Calculator

Convert a 48-bit MAC address into a modified EUI-64 IPv6 interface identifier. See the inserted FF:FE bytes, U/L bit inversion, interface ID, link-local address and an IPv6 address built from your own /64 prefix.

✓ MAC → EUI-64 ✓ FF:FE Insertion ✓ U/L Bit Flip ✓ Interface ID ✓ FE80::/64 ✓ Custom /64 Prefix

EUI

IPv6 Modified EUI-64 Calculation

● Ready

48-bit MAC Address Accepted formats include colon-separated, hyphen-separated, dotted Cisco-style and continuous 12-digit hexadecimal.

IPv6 /64 Prefix Optional custom /64 prefix used to create a complete IPv6 address.

Hex Display

IPv6 Display

Modified EUI-64 rule: insert FF:FE between the first three and last three MAC octets, then toggle bit 1 of the first octet using XOR 0x02. This is sometimes described as flipping the universal/local bit.

IPv6 EUI-64 Result Calculated

IPv6 Interface Identifier —

Normalized MAC —

Original First Octet —

Modified First Octet —

U/L Bit Before —

U/L Bit After —

Inserted Bytes —

EUI-64 Bytes —

Interface ID —

Link-Local IPv6 —

Custom IPv6 —

MAC U/L Type —

Result Status —

Step-by-Step Calculation —

Bit-Level U/L Flip —

IPv6 Address Forms —

Validation & Notes —

What Is IPv6 Modified EUI-64?

Modified EUI-64 is a method for deriving a 64-bit IPv6 interface identifier from a 48-bit Ethernet MAC address. The 48-bit address is expanded to 64 bits by inserting the two octets FF and FE in the middle.

The universal/local bit of the first MAC octet is then inverted to produce the modified EUI-64 form used by this calculation.

MAC Address to EUI-64 Formula

MAC:

AA:BB:CC:DD:EE:FF


Step 1:
Split after first 3 bytes

AA:BB:CC
DD:EE:FF


Step 2:
Insert FF:FE

AA:BB:CC:FF:FE:DD:EE:FF


Step 3:
Flip U/L bit of first octet

Modified first octet =
AA XOR 02

Example: 00:1A:2B:3C:4D:5E

Original MAC:
00:1A:2B:3C:4D:5E

Insert FF:FE:
00:1A:2B:FF:FE:3C:4D:5E

First octet:
00

Flip bit 1:
00 XOR 02
= 02

Modified EUI-64:
02:1A:2B:FF:FE:3C:4D:5E

IPv6 Interface ID:
021A:2BFF:FE3C:4D5E

Why Is FF:FE Inserted?

A 48-bit MAC contains six octets while an EUI-64 identifier contains eight. The conversion expands the original identifier by placing the fixed bytes FF and FE between its two three-octet halves.

48-bit MAC:

A B C | D E F

EUI-64:

A B C | FF FE | D E F

What Is the U/L Bit?

The U/L bit is bit 1 of the first MAC octet. In a MAC address it distinguishes universal and locally administered addressing conventions.

First MAC Octet:

bit 7              bit 0
 ↓                    ↓
00000000
      ↑
      U/L bit = bit 1

Mask:
00000010
= 0x02

Modified EUI-64 toggles this bit using XOR 0x02.

How the U/L Bit Is Flipped

Original first byte:
00

Binary:
00000000

XOR:
00000010

Result:
00000010

Hex:
02

If that bit is already one, toggling it changes it back to zero.

IPv6 Link-Local Address from EUI-64

A 64-bit interface identifier can be combined with the FE80::/64 link-local prefix.

Interface ID:

021A:2BFF:FE3C:4D5E

Link-local prefix:

FE80::/64

Result:

FE80::21A:2BFF:FE3C:4D5E

Custom IPv6 /64 Prefix

The same 64-bit interface identifier can be appended to a different /64 network prefix when that addressing method is appropriate.

Prefix:

2001:DB8:1234:5678::/64

Interface ID:

021A:2BFF:FE3C:4D5E

Result:

2001:DB8:1234:5678:021A:2BFF:FE3C:4D5E

EUI-64 Interface ID Size

Component	Bits	Bytes
Original MAC	48	6
Inserted FF:FE	16	2
Modified EUI-64	64	8
IPv6 Network Prefix	64	8
IPv6 Address	128	16

IPv6 EUI-64 vs Privacy Addresses

An EUI-64-derived interface identifier can reveal a stable relationship to a network interface identifier. Modern IPv6 systems may instead use stable privacy identifiers, temporary addresses or other address-generation methods.

Therefore, a MAC-derived EUI-64 calculation is useful for protocol study, legacy configurations, embedded systems, network troubleshooting and manual address calculations, but it should not be assumed that every modern IPv6 host actually uses a MAC-derived interface identifier.

Accepted MAC Address Formats

The calculator accepts several common representations and normalizes them before performing the conversion.

Colon:
00:1A:2B:3C:4D:5E

Hyphen:
00-1A-2B-3C-4D-5E

Cisco dotted:
001A.2B3C.4D5E

Continuous:
001A2B3C4D5E

IPv6 EUI-64 Calculator FAQs

How do I convert a MAC address to IPv6 EUI-64?

Split the 48-bit MAC after the first three octets, insert FF:FE, and toggle bit 1 of the first octet using XOR 0x02.

Why is FF:FE added to a MAC address?

The two bytes expand the six-byte MAC into an eight-byte EUI-64 identifier.

Which bit is flipped for modified EUI-64?

Bit 1 of the first octet, represented by mask 0x02, is toggled.

What is the EUI-64 for 00:1A:2B:3C:4D:5E?

The modified EUI-64 bytes are 02:1A:2B:FF:FE:3C:4D:5E, giving interface ID 021A:2BFF:FE3C:4D5E.

What is the link-local address for that interface ID?

Using FE80::/64, the resulting address is FE80::21A:2BFF:FE3C:4D5E in compressed form.

Does every IPv6 device use EUI-64?

No. Modern IPv6 implementations can use other interface identifier generation methods, including privacy-oriented or stable non-MAC-derived identifiers.

Can I use a custom /64 prefix?

Yes. The calculator can combine the generated 64-bit interface ID with a valid IPv6 /64 prefix.

Is EUI-64 the same as the original MAC address?

No. The original MAC is 48 bits. Modified EUI-64 is 64 bits and also changes the first octet’s U/L bit.