If you are looking for continuous monitoring of ports for server machines or local I think you can also use graphical version of nmap i.e Zenmap for more detailed version. Zenmap is the official graphical user interface (GUI) for the Nmap Security Scanner. Supports available (Linux, Windows, Mac OS X, BSD, etc.) Have a look to this tool view:

Open Port Check Tool This is a free utility for remotely verifying if a port is open or closed. It is useful to users who wish to verify port forwarding and check to see if a server is running or a firewall or ISP is blocking certain ports. Port Checker - Port Forwarding Test - Open Port Checker 26 rows Port Forwarding: How to Check Your Router for Open Ports

Re: Ports not opening after being forwarded. Apart from setting up port forwarding on the router, there are two other important requirements to fulfill before a port test will report success. The router's Internet/WAN port must have a public IP address.

An alternative to telnet in checking ports is Network Utility. In order to check ports on a Mac, follow the plan below: Open "Network Utility" > Click "Port Scan" > Indicate the hostname and ports to scan the remote host e.g. myserver.com from 995 to 995 > Check …

Below are possible port forwarding issues listed respectively from most to least common. Verify the router's WAN has a public IP address. Note that the connection may be NAT'd, and the address seen from the Web (ie: whatismyip.com) may not be the address on the WAN of the Cradlepoint.This is especially common for LTE / 3G connections.

How to Check for Ports in Use in Windows 10. By Robert Zak / Feb 15, 2019 / Windows. At any one time, there’s a whole bunch of information being sent between your Windows 10 PC and the endless void of the Internet. This is done using a process whereby network-dependent processes seek out free TCP and UDP ‘ports’ through which they