how to check open ports in my firewall

Question

I have an internet connection from school, where there are restriction about what ports webapp can use. Is it possible, to use my computer to check what ports are allowed to use and can be used? I'm having problem with websockets in couple of locations, so I would like to use client computer to do that. Is there any snippet of code that I can run in chrome console or any command line command?

Answer 1

nmap

Nmap ("Network Mapper") is a free and open source (license) utility for network exploration or security auditing. Many systems and network administrators also find it useful for tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime

From nmap: This paper details many of the techniques used to determine what ports (or similar protocol abstraction) of a host are listening for connections

Answer 2

I've always liked ShieldsUP! for checking open ports.