Network

netstat

Linux

$ netstat -anp

-a: Show both listening and non-listening (for TCP this means established connections) sockets. -n: Show numerical addresses instead of trying to determine symbolic host, port or user names. -p: Show the PID and name of the program to which each socket belongs.

Mac

$ lsof -n -i

-i: displays all open network connections and the name of the process that is using the connection. Adding a 4, as in -i4, will display only IPv4 connections. Adding a 6 instead (-i6) will display only IPv6 connections. The -i flag can also be expanded to specify further details. -iTCP or -iUDP will only return TCP and UDP connections. -iTCP:25 will only return TCP connections on port 25. A range of ports can be specified with a dash, as it -iTCP:25-50. Using -i@1.2.3.4 will return only connections to the IPv4 address 1.2.3.4. IPv6 addresses can be specified in the same fashion. The @ precursor can also be used to specify hostnames in the same way, but both remote IP addresses and hostnames cannot be used simultaneously.

-P: disables the conversion of port numbers to port names, speeding up output.

-n: disables the conversion of network numbers to host names. When used with -P above, it can significantly speed up lsof's output.

-u: user only returns commands owned by the named user.

Source: How to Use the Netstat Command on Mac