How do I see which processes have open TCP/IP ports in Mac OS X?

I use the command below when I want to see everything that's on a specific port for either TCP or UDP. The -n option disables attempting to resolve the IP addresses into domain names, and the -P disables attempting to figure out the name of a particular port. Also, running as root will show you more processes than running as a normal user.

sudo lsof -iTCP:53 -iUDP:53 -n -P

If you are interested in a specific port, you can use this example:

lsof -i 4tcp:8080 -sTCP:LISTEN

If you would only like to get the process id, you can run this:

lsof -i 4tcp:8080 -sTCP:LISTEN -Fp

The following code example lists all running TCP servers on your local macOS machine:

netstat -a -Ptcp | egrep 'tcp4.*LISTEN'

tcp4       0      0  127.0.0.1.2022         *.*                    LISTEN               0          0
tcp4       0      0  *.3141                 *.*                    LISTEN               0          0

LISTEN shows only sockets listening for connections. That is, servers.

The first line shows a server bound to localhost, aka 127.0.0.1, port 2022. It will answer to local requests, but not Internet-based requests.

The second line is a server bound to all addresses, ie *, port 3141. It will answer Internet queries.

To list ports used by clients and servers, use the following:

netstat -an -Ptcp | grep tcp4

This should be possible in a terminal window using the Netstat command.

And if you like the GUI way more:

With Mac OS X 10.5, the /Applications/Utilities folder contains a network utility called: Network Utility, see tab Netstat for these stats presented in a gui application, along with Ping, Lookup, Traceroute, Whois, Finger and Port Scan.