One address, millions of machines.

1981: IPv4 ships with 32-bit addresses — 4,294,967,296 of them. On the internet of that year, an unimaginable surplus.

3 February 2011: the last five /8 blocks leave IANA, one per regional registry. The global free pool is empty — permanently.

Then the registries drain, in order: APNIC 2011, RIPE NCC 2012, LACNIC 2014, ARIN 2015. Addresses now move only by trade — at roughly $27.75 each.

Demand never noticed. Eight billion people (Nov 2022), seventeen connected devices per US home. Machines outnumber addresses several times over — and everything still connects.

That is 0.54 addresses per person — computed on this page — before counting a single gadget. The fix, unchanged since 1994: a 16-bit field nobody talks about.

Every conversation is four numbers: your address, your slot, their address, their slot. The slot field is 16 bits — one address owns 65,536 of them, twice over (once for TCP, once for UDP).

The bottom rows are reserved: ports 0–1023, where services listen on fixed numbers — 443 for HTTPS, 53 for DNS.

Clients pick from the top quarter — 49152–65535, the dynamic range — a free slot rented for one conversation and returned.

This one — port 51,724, positioned by the code on this page — is your laptop's slot. The whole trick ahead is renting this wall out, machine by machine.

Your laptop sends FROM 192.168.1.20:51724 TO 93.184.216.34:443. Private addresses cannot cross the internet — so the router does something to this packet.

It rewrites the source to 203.0.113.7:50123 — its own public address plus a free slot — and stamps a row into its translation table. The table, not the address, now remembers who is talking.

The server replies to 203.0.113.7:50123 — the only address it has ever seen. The router matches the row, swaps the destination back, delivers.

Your phone talks too, from 192.168.1.34:40001: same public address, next free slot. Two conversations, two rows, one address.

From outside, your whole household is one machine — one address, a scatter of slots. That is why every device at home shows the same “what is my IP” answer.

The rows are rentals: they expire when quiet and vanish on reboot. Which is where the trouble starts.

A stranger sends to 203.0.113.7:50200 — a slot nobody claimed. No row, no match, no delivery: dropped. The bin's counter is computed from this page's scripted attempts. This is why you cannot host a game server by asking nicely.

Rows also expire: quiet UDP dies in ~30 seconds, established TCP can live ~5 days (netfilter defaults, illustrative). Idle lobbies disconnect because their rows evaporated — not because the network broke.

The exception is a row you write yourself. Port forwarding: slot 25565 always leads to the Minecraft machine, no conversation required — an invitation pinned to the door.

And every row is memory. A wire would never need this scene: NAT makes the middle of the network a party to every conversation — the elders call it breaking the end-to-end principle.

Your ISP runs the same ledger, city-sized. To it, your whole household is one endpoint: 100.64.12.8, from 100.64.0.0/10 — a private stage reserved in 2012 for exactly this.

The ISP's router rewrites again: your house leaves as 62.204.7.9:32001 — a public address you share with strangers, wearing a slot that is yours only for now.

Neighbors stack into the same table: one outside address, distinct slots. Operators must log every row — when abuse comes from a shared address, the log is the only way home.

Two clerks, no shared notes: double NAT. You control the home clerk; you will never control the ISP's. Its rules — its port rations — are next.

Two homes, two tables, both refusing inbound. First, discovery: each side asks a public STUN broker what it looks like from outside. You: 62.204.7.9:32044. Friend: 198.51.100.4:61200.

Now the punch: both fire outbound at each other, simultaneously. Each router stamps a row before the other's packet arrives — the hole exists on both walls.

The rows line up; the conversation runs direct, full speed. This choreography — standardized as ICE — is how voice, games and WebRTC mostly work behind NAT.

When tables are too strict, traffic falls back to a TURN relay — slower, always works. Every call you have ever made quietly picked one of these paths.