IPv6 Multicast Calculator
Build an IPv6 multicast address from a scope and group ID, and decode what the flag and scope fields mean.
Inputs
Multicast Address
ff15::1234
Scope Meaning
Site-local — bounded by the site
Flags Nibble
0x1 (transient)
Ethernet MAC Mapping
33:33:00:00:12:34
Step by step
Values used
Scope = 5 — Site-local; Group ID (hex) = 1234; Transient (dynamically assigned) group = Yes
IPv6 Multicast
A multicast address is ff, then a 4-bit flags nibble, then a 4-bit scope, then the group ID; the Ethernet destination is 33:33 followed by the last 32 bits.
Multicast Address
= ff15::1234
Scope Meaning
= Site-local — bounded by the site
Flags Nibble
= 0x1 (transient)
Ethernet MAC Mapping
= 33:33:00:00:12:34
How it works
Every IPv6 multicast address starts with ff. The flags nibble marks whether the group is permanently assigned, the scope nibble bounds propagation, and the remaining bits identify the group. Layer 2 delivery uses 33:33 plus the low 32 bits of the address. Scope selection decides whether multicast stays on a VLAN or floods a WAN, and the 33:33 mapping is what you match when hunting multicast storms in a capture.
Formula
IPv6 Multicast
A multicast address is ff, then a 4-bit flags nibble, then a 4-bit scope, then the group ID; the Ethernet destination is 33:33 followed by the last 32 bits.
- flags
- 0 for IANA-assigned, 1 for transient groups
- scope
- How far the traffic is allowed to propagate
Frequently Asked Questions
How is IPv6 Multicast calculated?
A multicast address is ff, then a 4-bit flags nibble, then a 4-bit scope, then the group ID; the Ethernet destination is 33:33 followed by the last 32 bits. Every IPv6 multicast address starts with ff. The flags nibble marks whether the group is permanently assigned, the scope nibble bounds propagation, and the remaining bits identify the group. Layer 2 delivery uses 33:33 plus the low 32 bits of the address.
Why does IPv6 Multicast matter?
Scope selection decides whether multicast stays on a VLAN or floods a WAN, and the 33:33 mapping is what you match when hunting multicast storms in a capture.
What values do I need to enter?
This calculator takes 3 inputs: Scope, Group ID (hex), Transient (dynamically assigned) group. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.