Work out nat pool size instantly with clear inputs, formula shown and shareable results.
Each NAT translation consumes one source port on a public address, so pool size is total concurrent sessions divided by the usable ports per address. Ports 0-1023 are normally excluded and many platforms reserve more, which is why 64512 rather than 65536 is the realistic figure. Running the pool near 100 percent port utilisation causes allocation failures that look like random connection resets.
NAT pool sizing
translations = hosts x sessions per host; public IPs = ceil(translations / usable ports per IP)
One address caps you at roughly 64,000 simultaneous translations, and per-host port limits reduce that further. Busy networks with many browser tabs and app sockets exhaust it quickly.
Web pages opening dozens of parallel connections, peer-to-peer software, mobile apps with persistent sockets, and short TIME_WAIT recycling all inflate the concurrent count.