netstat

Netstat prints information about the Linux networking subsystem. Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.
From man page

netstat --help

Here are some useful flags, such as -t we get the active internet connections that use the TCP protocol.

netstat -t

or the UDP protocol.
netstat -u

You can also see all the process with ports that are currently listening. Who is making contact with my server?

netstat -l

As an example, we can see what are the processes (p) that have open ports (l) with the IPv4 protocol (t4).

netstat -tl4p