Need a list of in-use higher socket numbers

[Dan Bernstein]

Past the low-number range allocated by the IETF, there are no real
standards. BSD systems define IPPORT_USERRESERVED in <netinet/in.h>,
50000 on some machines and 5000 on others; if you ask the system for a
port by binding to port 0, you'll get something lower than USERRESERVED.
So for maximum portability, you should pick a number above 50000.

It would be better if there were a central authority that allocated
nonstandard ports; I believe the IETF has refused to do this. Another
solution would be to have each machine translate services into ports,
through some sort of yellow pages; but this doesn't solve the namespace
management problem.

---Dan