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ARP Spoofing Risk Calculator

Score ARP poisoning exposure in a VLAN from host count, dynamic ARP inspection coverage, port security and cache timeout.

About ARP Spoofing Risk Calculator

Dynamic ARP inspection is the only control that actually stops the attack, so it carries half the mitigation weight; port security and static entries limit it, and filtering gratuitous ARP removes the easiest delivery method. The size factor scales risk with the log of host count, because a bigger broadcast domain means more machines an attacker can sit between. ARP has no authentication at all, so a single unmanaged host on a flat VLAN can transparently intercept traffic for every other host in that VLAN — including the default gateway.

Formula

ARP Spoofing Risk

risk = (100 − mitigation coverage) × size factor, where mitigation = 0.5·DAI + 0.25·port security + 0.15·static share + 10 if gratuitous ARP is filtered.

DAI
Dynamic ARP inspection, which validates ARP against DHCP snooping bindings
size factor
log₂(hosts) ÷ 8 — larger broadcast domains offer more targets

Worked example

With the default values below, ARP Spoofing Risk Calculator returns ARP Spoofing Risk Score of 73.1. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hosts in the broadcast domainaccepts 2–8,000 hosts
250 hosts
Ports with dynamic ARP inspection enabledaccepts 0–100 %
30 %
Ports with port security enabledaccepts 0–100 %
45 %
Static ARP entries configuredaccepts 0–8,000 entries
6 entries
ARP cache timeoutaccepts 10–86,400 seconds
240 seconds
Gratuitous ARP filtered at the access layer
No

Results

ARP Spoofing Risk Score
73.1 /100
Risk Band
High
Unprotected Host Share
70.0%
Mitigation Coverage Score
26.6 /100
Hosts Reachable from One Foothold
249
Average Re-Poisoning Interval
120 seconds

Step by step

  1. 1Values used
  2. 2ARP Spoofing Risk
  3. 3ARP Spoofing Risk Score= 73.1 /100
  4. 4Risk Band= High
  5. 5Unprotected Host Share= 70.0
  6. 6Mitigation Coverage Score= 26.6 /100
  7. 7Hosts Reachable from One Foothold= 249
  8. 8Average Re-Poisoning Interval= 120 seconds

Assumptions and limits

  • The result is derived only from the 6 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 1 decimal place, 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 ARP Spoofing Risk calculated?

risk = (100 − mitigation coverage) × size factor, where mitigation = 0.5·DAI + 0.25·port security + 0.15·static share + 10 if gratuitous ARP is filtered. Dynamic ARP inspection is the only control that actually stops the attack, so it carries half the mitigation weight; port security and static entries limit it, and filtering gratuitous ARP removes the easiest delivery method. The size factor scales risk with the log of host count, because a bigger broadcast domain means more machines an attacker can sit between.

Why does ARP Spoofing Risk matter?

ARP has no authentication at all, so a single unmanaged host on a flat VLAN can transparently intercept traffic for every other host in that VLAN — including the default gateway.

What values do I need to enter?

This calculator takes 6 inputs: Hosts in the broadcast domain, Ports with dynamic ARP inspection enabled, Ports with port security enabled, Static ARP entries configured, ARP cache timeout, Gratuitous ARP filtered at the access layer. 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 the attacker have to keep re-poisoning?

ARP cache entries expire, typically after 4 minutes on Linux and up to 20 on Windows, and any legitimate ARP exchange overwrites the forged entry. An attacker therefore refreshes the poison roughly twice per cache lifetime, which is exactly the repetitive gratuitous ARP pattern that detection tooling looks for.

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