I imagine that's the limit per client IP address [for a single server port], no? The Linux kernel can use multiple pieces of information to track connections: client IP address, client port, server IP address, server port.

Cloudflare has some interesting blog posts on this topic:

- https://blog.cloudflare.com/how-we-built-spectrum/

- https://blog.cloudflare.com/how-to-stop-running-out-of-ephem...