How to find the open ports when you get connected

This post is all about the open ports created in your system. This ports are mainly tcp or udp ports. 

Ports below 1024 are reserved for common services, and only root can use them. Standard port numbers can be found in /etc/services. 

Take your terminal and type  in as follows to see them

$sudo gedit /etc/services

The maximum number of ports is 65k, so you have more than enough Internet ports for all your services. Here are some useful utilities. Netstat is a command that will list both the open ports and who is connected to your system. You should run it like this:

$netstat -an | more

This way you can find out who is connected to which service. Another interesting command is the fuser program. This program can tell you which user and  process owns a port. For example, the following command will tell you who owns port 0

$fuser -v -n tcp 0