How could two processes bind onto the same port?

Matt Ball asked:

I just ran into an issue where a request made to localhost:8080 from curl was hitting a different server than the same request made from inside Node. lsof -i :8080 revealed that two processes were both binding onto the same port:

node    51961 mball   14u  IPv4 0xd980e0df7f175e13      0t0  TCP *:http-alt (LISTEN)
java    62704 mball  320u  IPv6 0xd980e0df7fe08643      0t0  TCP *:http-alt (LISTEN)

How is this possible? Were they binding onto different interfaces? Or was it the IPv4 vs 6?

If you’re curious, node was hitting the other node process, curl was hitting the java process. The java process was started after the node process.

My answer:

They may be the same port, but they are not the same address – or even the same address family! Yes, the IPv4 versus IPv6 matters.

To fix the problem, have the desired process bind to both IPv4 and IPv6 address families.

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