Hello,
how do you check some ip:port for availability from a mikrotik router?
I do not understand the question?
As an admin you have all the ports available for your use, just pick one. If you cant here is an idea 45699
Done!!
You do not understand the question.
Then ask a better question and provide some context.
What is the use case what is it that you are trying to achieve.
that you don’t understand?
You’re on a mikrotik router (for example via winbox).
Now you need to check for port availability at some address (for example 1.2.3.4:945 or 5.6.7.8:1843)
If it’s tcp and you want to do it manually, you can always try telnet:
/system telnet address=1.2.3.4 port=945
It’s not meant for checking ports, but you’ll see if it’s open or not.
Ahh, okay well that makes much more sense.
You want to scan ports on IPs on the internet like hackers and bots do! 
Fill your boots…
https://securitytrails.com/blog/best-port-scanners
Personally I use steve gibsons site…
https://www.grc.com/default.htm
and his Shields UpTest to check out my own router.
https://www.grc.com/x/ne.dll?bh0bkyd2
+1 please, add this features
I usually need to port scan some ridiculos DVR or printers or another stuff. And its annoing to make VPN connection for linux os, or come into local PC by temviewer to scan with nmap.
ROS is SO powerful, why it doesnt have so necessary network tool