EUI-64 Link-Local Calculator

About this interactive

When an interface turns IPv6 on, it makes its own link-local address: fe80 and three zero hextets for the network part, then a 64-bit host part, the interface ID. One way to build that host part is EUI-64, which stretches the interface's 48-bit MAC address to 64 bits. Split the MAC in half and put ff:fe in the middle; then invert the seventh bit of the first byte, the bit worth 2. For the MAC 00:25:96:12:34:56 that is 0025:96ff:fe12:3456 with 00 becoming 02, so the interface ID is 225:96ff:fe12:3456 and the link-local address is fe80::225:96ff:fe12:3456. The flip is an inversion, not an addition: a burned-in MAC has that bit 0, so it becomes 1 (00 to 02, a8 to aa), but a locally administered MAC already has it set, and the flip clears it (52 to 50). Because the steps undo cleanly, anyone who sees an EUI-64 address can read the MAC back out of it, which is why many operating systems now use a random interface ID instead (RFC 7217, RFC 8064), while routers commonly still use EUI-64. Before using the address, the host runs duplicate address detection to make sure no neighbour already has it. Seven of the twenty-one questions are drawn each run: some ask for the interface ID, some the whole link-local address, some a single hextet, and some run it backwards from an address to its MAC. Anything you miss shows the answer and how to work it out.

What you'll learn

Build the EUI-64 interface ID and the link-local address from a MAC address: insert ff:fe in the middle and invert the seventh bit
Recover the MAC address from a link-local address that EUI-64 built, by reversing the same steps

Aligned to

Cisco CCNA

1.9 Describe IPv6 address types

1.8 Configure and verify IPv6 addressing and prefix

Cisco CCST Networking

2.3 Identify IPv6 addresses and prefix formats

CompTIA Network+

3.4 Given a scenario, implement IPv4 and IPv6 network services

Key terms

Link-Local Address: An IPv6 address in the FE80::/10 range that is automatically assigned to every IPv6-enabled interface and used only for communication within a single network link. It cannot be routed beyond the local Layer 2 segment and is required for Neighbor Discovery and router communication.
Interface ID: The last 64 bits of an IPv6 address that uniquely identify a specific interface or node within a network, equivalent to the host portion in IPv4.
MAC Address: A 48-bit hardware address assigned to a network interface and written as six hexadecimal octets, such as 00:1A:2B:3C:4D:5E. It identifies a device uniquely on a local network segment, and switches forward frames by looking it up in their MAC address table.

Topics

Ipv6 Eui 64 Link Local Mac Address Interface Id Interactive Calculation

About TechKnowSurge

TechKnowSurge builds IT and cybersecurity professionals through hands-on, concept-first training built around real understanding — not memorization. Free interactive tools, structured programs, and 25+ years of real-world experience, all in one place.

Explore free tools and programs →