[HowTo] Asus RT-N16 (Oleg) - optware the simple way

[MercuryV]

How can I find out what uses port 80?

It's typically webserver serving firmware web interface.
But you can check this. For example, on my device it's nginx

$ netstat -nlp | grep ':80 '
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      579/nginx
$ which nginx
/opt/sbin/nginx