IPv6 by hand: address types, finding the prefix, EUI-64 and solicited-node multicast

A quick map of IPv6 address types

IPv6 has three kinds of address. Unicast identifies one interface; multicast identifies a group; anycast is a unicast address shared by several devices, and the network delivers to the nearest one. There is no broadcast.

The background to these types is in IPv6 explained; this note is about working with them by hand.

Finding the prefix

Each hex digit is 4 bits and each block of four digits is 16 bits. So for prefix lengths that are a multiple of 4 (a nibble boundary), keep that many bits' worth of digits and set the rest to zero. Take the host address 2001:db8:acad:1234:a1b2::9:

• /64 keeps four blocks: 2001:db8:acad:1234::/64. Almost every LAN subnet is a /64.
• /56 keeps three blocks and two digits of the fourth: 2001:db8:acad:1200::/56. Many ISPs give homes a /56 (256 /64 subnets).
• /48 keeps three blocks: 2001:db8:acad::/48. A typical site allocation (65,536 /64 subnets).

When the prefix is not on a nibble boundary, convert only the digit that is split. For 2001:db8:4c2a:7b31::25/62, the first 60 bits run to the third digit of the fourth block (7b3). The last digit, 1, is 0001 in binary; keep its first 2 bits (00) and zero the rest, giving 0000 = 0. The prefix is 2001:db8:4c2a:7b30::/62, which covers the four /64s from 7b30 to 7b33. Expand and compress addresses with the IPv6 tool to check your work.

EUI-64 step by step

A host or router can build the 64-bit interface ID from its 48-bit MAC address with modified EUI-64:

1. Split the MAC into two halves of 24 bits: 005056 and abcdef.
2. Insert fffe between them: 005056fffeabcdef.
3. Flip the seventh bit of the first byte, the universal/local (U/L) bit. The first byte 00 is 0000 0000; flipping the seventh bit gives 0000 0010 = 02. A quick trick: the second hex digit changes 0 to 2, 2 to 0, 4 to 6, 8 to a, c to e, and so on.
4. Write it in blocks: 0250:56ff:feab:cdef. With the prefix 2001:db8:acad:10::/64 the address is 2001:db8:acad:10:250:56ff:feab:cdef.

On a Cisco router you can ask for this automatically:

interface GigabitEthernet0/1
 ipv6 address 2001:db8:acad:10::/64 eui-64
 ipv6 enable

Routers often use EUI-64 or manual addresses. Desktop and mobile operating systems now usually prefer random or stable private interface IDs instead, because EUI-64 exposes the MAC address and lets a device be tracked from network to network.

Solicited-node multicast

IPv4 uses a broadcast ARP request, which disturbs every host on the subnet. IPv6 Neighbour Discovery sends its Neighbour Solicitation to a solicited-node multicast address instead, so only hosts whose address ends in the same 24 bits need to look at it. Every interface joins the solicited-node group for each of its unicast addresses.

1. Take the last 24 bits (six hex digits) of the unicast address. For 2001:db8:acad:12::a1b2:c3d4 they are b2:c3d4.
2. Add them to ff02::1:ff: the result is ff02::1:ffb2:c3d4.
3. On Ethernet, IPv6 multicast maps to a MAC address of 33:33 plus the last 32 bits of the group: 33:33:ff:b2:c3:d4. Switches and network cards can filter on it, so other hosts ignore the frame in hardware.

The full Neighbour Discovery process, including duplicate address detection, is in IPv6 Neighbour Discovery, SLAAC and DHCPv6.

Real-world example: documenting a branch IPv6 plan

A network engineer at a fictional retail chain received 2001:db8:5a00::/48 for a new regional office. She gave each floor a /56 and each VLAN a /64 inside it, so the second floor's voice VLAN became 2001:db8:5a00:210::/64. Router interfaces used short manual addresses ending in ::1 so they were easy to recognise, while the uplink to the core used EUI-64 for a quick lab test before being changed to a manual address. When a colleague asked why a packet capture showed traffic to ff02::1:ff00:1, she explained it was a neighbour solicitation for the router's ::1 address, sent to its solicited-node group, not a broadcast storm.