Why is nmap skipping ports?

Question

linux

Hello everyone again.
Locked extra ports, scanned with nmap, everything is ok, ports are filtered. I started a web server on port 300, I scan through nmap, and it says nothing at all about this port.
Why does he "forget" it? So I began to doubt that he would scan all the ports and I would block the excess.

Answer 1

By the way udp or tcp port? - what are the scan settings in nmap?
nmap -p 300 hostname - scan port 300
nmap -sT hostname - scan all tcp on the target host
and by the way what's with the firewall?
and a completely stupid question - but for sure the server hangs on the external interface, and not on 127.0.0.1

Answer 2

settings pick
by default checks standards only, up to 1024