Broadcast Domain Calculator
Split hosts into broadcast domains and check the broadcast bandwidth every NIC in a domain has to absorb.
About Broadcast Domain Calculator
Broadcast is the one traffic class that does not scale with switching capacity: every frame is delivered to every host in the domain and interrupts its CPU. Splitting hosts into more, smaller domains is the only way to reduce the load any individual host sees. This is the arithmetic behind the old "keep a subnet under 250 hosts" rule, and it is also how you pick a storm-control threshold that catches a real loop without tripping on a legitimate boot storm.
Formulas
Broadcast Domain
Domains = ceil(hosts ÷ hosts per VLAN); every host in a domain receives the whole domain's broadcast, so load = hosts per domain × broadcasts per host × frame bits.
- domains
- Number of VLANs or subnets required
- on wire
- Frame size plus the 20 bytes of preamble, SFD and interframe gap
- storm threshold
- Packets per second equal to 1% of the NIC's line rate
Storm-control sizing
storm-control threshold = 1% of line rate ÷ on-wire frame bits.
Worked example
With the default values below, Broadcast Domain Calculator returns Broadcast Domains Needed of 8. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total hostsminimum 1 hosts
- 2,000 hosts
- Target hosts per VLANaccepts 2–4,000 hosts
- 250 hosts
- Broadcasts per hostaccepts 0–500 pps
- 5 pps
- Average broadcast frameaccepts 64–1,518 bytes
- 64 bytes
- Host NIC speedminimum 1 Mbps
- 1,000 Mbps
Results
- Broadcast Domains Needed
- 8
- Hosts per Domain
- 250
- Broadcast Packets per Second per Domain
- 1,250 pps
- Broadcast Load per Host
- 0.840 Mbps
- Host NIC Consumed by Broadcast
- 0.084%
- Storm-Control Threshold at 1%
- 14,881 pps
Step by step
- 1Values used
- 2Broadcast Domain
- 3Storm-control sizing
- 4Broadcast Domains Needed= 8
- 5Hosts per Domain= 250
- 6Broadcast Packets per Second per Domain= 1,250 pps
- 7Broadcast Load per Host= 0.840 Mbps
- 8Host NIC Consumed by Broadcast= 0.084
- 9Storm-Control Threshold at 1%= 14,881 pps
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 3 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 Broadcast Domain calculated?
Domains = ceil(hosts ÷ hosts per VLAN); every host in a domain receives the whole domain's broadcast, so load = hosts per domain × broadcasts per host × frame bits. Broadcast is the one traffic class that does not scale with switching capacity: every frame is delivered to every host in the domain and interrupts its CPU. Splitting hosts into more, smaller domains is the only way to reduce the load any individual host sees.
Why does Broadcast Domain matter?
This is the arithmetic behind the old "keep a subnet under 250 hosts" rule, and it is also how you pick a storm-control threshold that catches a real loop without tripping on a legitimate boot storm.
What values do I need to enter?
This calculator takes 5 inputs: Total hosts, Target hosts per VLAN, Broadcasts per host, Average broadcast frame, Host NIC speed. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Do broadcast and multicast count the same?
For an unoptimised switch, yes — unknown multicast floods like broadcast. With IGMP snooping, multicast only reaches subscribed ports, so exclude it from the per-host figure once snooping is verified working.