Broadcast Domain Calculator
Split hosts into broadcast domains and check the broadcast bandwidth every NIC in a domain has to absorb.
Inputs
Broadcast Domains Needed
8
Hosts per Domain
250
Broadcast Packets per Second per Domain
1,250pps
Broadcast Load per Host
0.840Mbps
Host NIC Consumed by Broadcast
0.084%
Storm-Control Threshold at 1%
14,881pps
Step by step
Values used
Total hosts = 2,000 hosts; Target hosts per VLAN = 250 hosts; Broadcasts per host = 5 pps; Average broadcast frame = 64 bytes; Host NIC speed = 1,000 Mbps
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.
Storm-control sizing
storm-control threshold = 1% of line rate ÷ on-wire frame bits.
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
How it works
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.
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.