mDNS Calculator

Estimate multicast DNS packet rate, bandwidth and wireless airtime cost for a device population.

About mDNS Calculator

mDNS floods the whole layer-2 segment, so packet rate is the total record count divided by the refresh interval. On Wi-Fi the cost is far worse than the bit count suggests, because multicast is transmitted at the lowest basic rate and cannot be aggregated. A few hundred Bonjour devices on one flat wireless VLAN can consume double-digit percentages of airtime with nothing but announcements, which is why large deployments proxy mDNS instead of bridging it.

Formulas

mDNS

packet rate = devices × services per device ÷ announcement interval.

devices × services: Total advertised service records on the segment
announcement interval: Seconds between unsolicited announcements or record refreshes

Wireless airtime cost

airtime = packets/s × packet bits ÷ multicast basic rate

multicast basic rate: Multicast frames are sent at the lowest basic rate, often 1–6 Mbit/s
airtime: Milliseconds of channel time consumed per second

Worked example

With the default values below, mDNS Calculator returns Multicast Packet Rate of 6.25. Change any input above and every figure here is recalculated in your browser.

Inputs used

Devices advertising servicesminimum 1 devices: 250 devices
Services per deviceminimum 1 services: 3 services
Announcement intervalminimum 1 s: 120 s
Bytes per mDNS packetminimum 40 bytes: 200 bytes
Multicast basic rateminimum 0.1 Mbit/s: 1 Mbit/s

Results

Multicast Packet Rate: 6.25 pps
Multicast Bandwidth: 10.00 kbit/s
Airtime Consumed: 10.00 ms/s
Airtime Utilisation: 1.00%
Packets Per Minute: 375

Step by step

1Values used
2mDNS
3Wireless airtime cost
4Multicast Packet Rate= 6.25 pps
5Multicast Bandwidth= 10.00 kbit/s
6Airtime Consumed= 10.00 ms/s
7Airtime Utilisation= 1.00
8Packets Per Minute= 375

Assumptions and limits

The result is derived only from the 5 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 2 decimal places, 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

How is mDNS calculated?

packet rate = devices × services per device ÷ announcement interval. mDNS floods the whole layer-2 segment, so packet rate is the total record count divided by the refresh interval. On Wi-Fi the cost is far worse than the bit count suggests, because multicast is transmitted at the lowest basic rate and cannot be aggregated.

Why does mDNS matter?

A few hundred Bonjour devices on one flat wireless VLAN can consume double-digit percentages of airtime with nothing but announcements, which is why large deployments proxy mDNS instead of bridging it.

What values do I need to enter?

This calculator takes 5 inputs: Devices advertising services, Services per device, Announcement interval, Bytes per mDNS packet, Multicast basic rate. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Why does multicast cost so much airtime?

Access points send multicast at the lowest configured basic rate so every associated client can decode it, and without a unicast acknowledgement there is no aggregation. A 200-byte frame at 1 Mbit/s occupies roughly 1.6 ms of channel time — hundreds of times what the same frame costs at modern data rates.