Network Fundamentals · Topic 17 of 17

IPv6 Address Types and EUI-64 Practice

Worked practice for recognising IPv6 address types from their first digits and building interface IDs with EUI-64, link-local and solicited-node addresses.

5 min

In a nutshell

Global unicast starts with 2 or 3, link-local with FE80, unique local with FC or FD.
Multicast starts with FF. FF02::1 is all nodes and FF02::2 is all routers.
IPv6 has no broadcast.

What it is

Two IPv6 skills reward practice: telling address types apart at a glance, and building an interface ID from a MAC address.

This page drills both with worked steps. The method is the same as on the address types page, so use that page for the theory.

Work each problem on paper, then compare.

How it works

Read the first digits to find the type, and follow three steps to build an EUI-64 interface ID.

Problem 1: name the type

Address	Type	How you can tell
2001:DB8::1	Global unicast	Starts with 2
3001:DB8::1	Global unicast	Starts with 3, inside 2000::/3
FE80::1	Link-local	FE80::/10
FD12:3456:789A::1	Unique local	FC00::/7, and FD in practice
FF02::1	Multicast, all nodes	FF00::/8
FF02::2	Multicast, all routers	FF00::/8
::1	Loopback	Only the last bit is 1
::	Unspecified	All zeros

The bit flip in one line

The flip changes the second hex digit of the first byte. Digits 0, 1, 4, 5, 8, 9, C and D add 2. Digits 2, 3, 6, 7, A, B, E and F subtract 2.

Exam tip

For a first byte of 00, 10, 40 or any byte ending in 0, the result ends in 2. A first byte of 02 becomes 00.

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

Split in the middle: 00:1A:2B and 3C:4D:5E.
Insert FFFE: 001A:2BFF:FE3C:4D5E.
Flip the bit: the first byte 00 becomes 02.

Interface ID: 021A:2BFF:FE3C:4D5E
With prefix 2001:DB8:1:1::/64: 2001:DB8:1:1:21A:2BFF:FE3C:4D5E
Link-local: FE80::21A:2BFF:FE3C:4D5E

Problem 3: MAC 02:00:5E:10:00:01

Splitting and inserting FFFE gives 0200:5EFF:FE10:0001. The first byte is 02, and its second digit 2 subtracts 2 to give 0.

Interface ID: 0000:5EFF:FE10:0001
With prefix 2001:DB8:1:1::/64: 2001:DB8:1:1:0:5EFF:FE10:1

Problem 4: MAC A4:BB:6D:12:34:56

The raw ID is A4BB:6DFF:FE12:3456. The first byte A4 has second digit 4, which adds 2 to give 6.

Interface ID: A6BB:6DFF:FE12:3456

Problem 5: MAC FC:99:47:75:CE:E0

The raw ID is FC99:47FF:FE75:CEE0. The first byte FC has second digit C, which adds 2 to give E.

Interface ID: FE99:47FF:FE75:CEE0

Problem 6: work backwards

Given the interface ID 0211:22FF:FE33:4455, find the MAC.

Flip the bit back: the first byte 02 becomes 00.
Remove FFFE from the middle.
Rejoin the bytes: 00:11:22 and 33:44:55 give 00:11:22:33:44:55.

MAC: 00:11:22:33:44:55

Problem 7: the solicited-node address

A solicited-node multicast address is FF02::1:FF00:0/104 plus the last 24 bits of the unicast address.

The address FE80::21A:2BFF:FE3C:4D5E ends in 3C:4D5E, so its solicited-node address is FF02::1:FF3C:4D5E.

Watch out

Use the last 24 bits, six hex digits, not the last 16. The six digits 3C4D5E are 24 bits, while the last group 4D5E is only 16.

Problem 8: let the router build the address

On an interface whose MAC address is 00:50:56:AA:BB:CC, the command below builds the interface ID with EUI-64.

interface GigabitEthernet0/0
 ipv6 address 2001:DB8:1:1::/64 eui-64

The interface ID is 0250:56FF:FEAA:BBCC, so show ipv6 interface brief lists 2001:DB8:1:1:250:56FF:FEAA:BBCC.

Problem 9: which of these can cross the internet?

Only global unicast addresses are routed on the public internet.

FE80::1 is link-local, so it never leaves its own link.
FD00::1 is unique local, so it is private to an organisation.
2001:DB8:1::1 has the global unicast format, but the 2001:DB8::/32 range is reserved for documentation and is not used on real networks.

So none of the three can cross the internet. A real global unicast address, assigned by a provider from the 2000::/3 range, can.

What to remember for the exam

Seven points are worth memorising.

Global unicast starts with 2 or 3, link-local with FE80, unique local with FC or FD.
Multicast starts with FF. FF02::1 is all nodes and FF02::2 is all routers.
IPv6 has no broadcast.
EUI-64 splits the MAC, inserts FFFE, and flips bit 7 of the first byte.
The flip changes the second hex digit of the first byte by 2.
A link-local address is FE80:: plus the interface ID.
A solicited-node address uses the last 24 bits of the unicast address.

Common mistakes

These four errors recur.

Flipping the wrong bit. It is the seventh bit of the first byte, which changes a 00 into 02, not a 00 into 80.

Forgetting FFFE. The MAC has 48 bits and the interface ID needs 64.

Treating ::1 as global. It is the loopback address.

Using 16 bits for the solicited-node address. It takes 24.

Check yourself

Try these three, then click a question to reveal its answer.

What type of address is FD00:1::5?
Unique local.
What is the EUI-64 interface ID for MAC 00:50:56:AA:BB:CC?
0250:56FF:FEAA:BBCC.
Which MAC address gives the interface ID 0000:5EFF:FE10:0001?
02:00:5E:10:00:01.

Topic 17 of 17 in Network Fundamentals

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