14 / 14 IP & Addressing

Depends How It Keys The Mapping

A carrier-grade NAT fronts 5,000 subscribers behind one public IPv4 address. Most of them hold connections open to the same popular service at 203.0.113.10:443, and each also talks to a few dozen other destinations. Which statement about the simultaneous-TCP-connection ceiling is right?

A

There is no ceiling. NAT rewrites only the source IP address and leaves ports untouched, which is why the translation table is keyed on addresses.

B

With endpoint-independent mapping, the NAT commits one external port per internal source endpoint regardless of destination, so the whole box tops out near the size of the port space. A NAT that keys the mapping on the full destination address and port can reuse one external port across different destinations and carry far more flows, at the cost of the behaviour REQ-1 exists to preserve.

C

The ceiling is 65,536 per subscriber, since the NAT gives each internal host its own port space and only rewrites the source address.

D

The ceiling applies per destination, so 5,000 subscribers can all reach 203.0.113.10:443 and every other server without ever exhausting ports.