IPv6 Interface ID Calculator
Split an IPv6 address into its network prefix and 64-bit interface identifier, with the identifier type flagged.
About IPv6 Interface ID Calculator
Splitting at the prefix boundary separates the routed portion from the host portion. Addresses built by SLAAC from a MAC keep the ff:fe signature; RFC 4941 privacy addresses and static assignments do not. An interface identifier that encodes a MAC address leaks hardware vendor and tracks a device across networks, which is exactly what privacy extensions exist to prevent.
Formula
IPv6 Interface ID
The interface identifier is the low 128 − prefix bits; an EUI-64 identifier carries the fixed ff:fe pattern in the middle.
- p
- Prefix length
- ff:fe
- Marker inserted when a 48-bit MAC is stretched to 64 bits
Worked example
With the default values below, IPv6 Interface ID Calculator returns Interface Identifier of 021a:2bff:fe3c:4d5e. Change any input above and every figure here is recalculated in your browser.
Inputs used
- IPv6 address
- 2001:db8:acad:1:021a:2bff:fe3c:4d5e
- Prefix lengthaccepts 0–128 bits
- 64 bits
Results
- Interface Identifier
- 021a:2bff:fe3c:4d5e
- Network Prefix
- 2001:db8:acad:1::/64
- Identifier Type
- EUI-64 derived from a MAC address
- Identifier Bits
- 64
Step by step
- 1Values used
- 2IPv6 Interface ID
- 3Interface Identifier= 021a:2bff:fe3c:4d5e
- 4Network Prefix= 2001:db8:acad:1::/64
- 5Identifier Type= EUI-64 derived from a MAC address
- 6Identifier Bits= 64
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is IPv6 Interface ID calculated?
The interface identifier is the low 128 − prefix bits; an EUI-64 identifier carries the fixed ff:fe pattern in the middle. Splitting at the prefix boundary separates the routed portion from the host portion. Addresses built by SLAAC from a MAC keep the ff:fe signature; RFC 4941 privacy addresses and static assignments do not.
Why does IPv6 Interface ID matter?
An interface identifier that encodes a MAC address leaks hardware vendor and tracks a device across networks, which is exactly what privacy extensions exist to prevent.
What values do I need to enter?
This calculator takes 2 inputs: IPv6 address, Prefix length. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.