ARP Cache Calculator
Calculate ARP cache size and expiration behavior based on neighbor table thresholds.
About ARP Cache Calculator
The ARP (or IPv6 neighbor discovery) cache maps IP addresses to MAC addresses for hosts on the same local subnet, growing roughly in proportion to the number of distinct neighbors a host has actually communicated with recently, up to gc_thresh limits. Entries age out after gc_stale_time seconds of inactivity (triggering a re-resolution probe on next use rather than immediate deletion), and each entry carries a small but nonzero kernel memory cost — on subnets with very high host counts (large flat L2 networks), ARP cache sizing and gc_thresh tuning become relevant to avoid table exhaustion warnings.
Formula
ARP cache entries
entries = hosts_on_subnet (bounded by neigh gc_thresh limits)
- H_{subnet}
- distinct hosts recently communicated with on the subnet
Worked example
With the default values below, ARP Cache Calculator returns Estimated ARP Cache Entries of 180. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Hosts on Subnet (active neighbors)minimum 0
- 180
- gc_stale_time (seconds)minimum 1
- 60
- Memory per ARP Entry (bytes)minimum 1
- 100
Results
- Estimated ARP Cache Entries
- 180
- Estimated Cache Memory (KB)
- 17.58
- Stale Timeout (seconds)
- 60
- Est. Total Re-resolutions per Hour
- 10,800
Step by step
- 1Values used
- 2ARP cache entries
- 3Estimated ARP Cache Entries= 180
- 4Estimated Cache Memory (KB)= 17.58
- 5Stale Timeout (seconds)= 60
- 6Est. Total Re-resolutions per Hour= 10,800
Assumptions and limits
- The result is derived only from the 3 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 do I view and check the size of the ARP cache on Linux?
`ip neigh show` lists all current neighbor table entries with their state (REACHABLE, STALE, etc.); `cat /proc/net/arp | wc -l` gives a quick count (subtract 1 for the header row).
What happens when the ARP cache hits its gc_thresh3 limit?
The kernel logs 'neighbour table overflow' and drops the oldest/least-recently-used entries to make room, which can cause brief connectivity hiccups on very large flat networks — raising `net.ipv4.neigh.default.gc_thresh1/2/3` is the standard fix for legitimately large subnets.
Why does an entry go STALE rather than being deleted immediately at gc_stale_time?
Marking an entry STALE (rather than deleting it outright) lets the kernel keep using the cached MAC address optimistically while probing to confirm it's still valid, avoiding a lookup stall on every single packet — only entries that fail to be confirmed and haven't been used are eventually garbage collected.