IPv6 Interface ID Calculator
Split an IPv6 address into its network prefix and 64-bit interface identifier, with the identifier type flagged.
Inputs
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
Values used
IPv6 address = 2001:db8:acad:1:021a:2bff:fe3c:4d5e; Prefix length = 64 bits
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.
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
How it works
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
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.