Multicast MAC Mapping Calculator

Work out multicast mac mapping instantly with clear inputs, formula shown and shareable results.

About Multicast MAC Mapping Calculator

IPv4 multicast frames use the 01:00:5e OUI with the low 23 bits of the group address copied into the remaining bits. Because a group has 28 significant bits, five are discarded and 32 different groups collide on one MAC — so a NIC filtering only on MAC may hand unwanted groups to the kernel. IPv6 uses 33:33 followed by the low 32 bits of the group.

Formulas

IPv4 group to MAC

MAC = 01:00:5e:(bit23..bit16 of group with top bit cleared):(bit15..8):(bit7..0)

IPv6 group to MAC

MAC = 33:33:(last four bytes of the IPv6 group address)

Worked example

With the default values below, Multicast MAC Mapping Calculator returns Destination multicast MAC of 01:00:5e:01:02:03. Change any input above and every figure here is recalculated in your browser.

Inputs used

Multicast group address: 239.1.2.3
Address familyone of IPv4 (01:00:5e prefix), IPv6 (33:33 prefix): IPv4 (01:00:5e prefix)

Results

Destination multicast MAC: 01:00:5e:01:02:03
Bits carried into the MAC: low 23 bits = 0x10203
Groups sharing this MAC: 32
Why aliasing happens: Five of the 28 group bits are discarded, so 32 IPv4 groups map onto the same MAC address

Step by step

1Values used
2IPv4 group to MAC
3IPv6 group to MAC
4Destination multicast MAC= 01:00:5e:01:02:03
5Bits carried into the MAC= low 23 bits = 0x10203
6Groups sharing this MAC= 32
7Why aliasing happens= Five of the 28 group bits are discarded, so 32 IPv4 groups map onto the same MAC address

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why do 32 groups share one MAC?

Only 23 of the 28 group bits fit in the reserved MAC space, so groups differing solely in those five high bits produce identical destination MACs.

Does aliasing cause real problems?

It can. A host subscribed to one group may receive frames for an aliased group and burn CPU discarding them at the IP layer, which matters on high-rate feeds.