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I check my opened ports via Network Utility (macOS X High Sierra 10.3.3) and get this result:

Port Scanning host: MY EXTERNAL IP

 Open TCP Port:     21          ftp
 Open TCP Port:     23          telnet
 Open TCP Port:     80          http
 Open TCP Port:     1900        ssdp
 Open TCP Port:     5916
 Open TCP Port:     7547        cwmp
 Open TCP Port:     20005
 Open TCP Port:     33344
 Open TCP Port:     56614

But if I try to check the same ports online, through a website port checker tool on my external ip, the result is all these ports closed or connection timed out... what is the difference? so what is the REAL result for my TP-LINK router and system? are these ports safe, if opened? and generally which are the only ones safe (if opened) for macos X?

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    Are you on your local network when you check your ports? You can't scan your external network ports from inside the same network. You might have those ports open on your Mac (as reported by Network Utility) but your firewall/router has those ports closed. Now, as to those particular ports - why, oh why, do you have ftp and telnet enabled??? They are horribly insecure. You should be using ssh (port 22) instead.
    – Allan
    Commented Feb 15, 2018 at 11:51
  • Oh yes @Allan, I'm on my local network (internal address 192.168.X.XXX, even if I scanned my external ip address), so the REAL result is the website port checker tool? About ftp and telnet enabled I think its a function of TP-LINK Router (USB port in the back, and storage across the network)
    – SILminore
    Commented Feb 15, 2018 at 12:00
  • Those are the management ports on your firewall. It has two IPs (the public and the private). If you attempt to connect to it via the public IP, it will not go out, circle back, and then connect, it will connect to the private side. What these results are telling you is that those ports are open inside your network and closed outside. That said...they're still insecure. Both results are correct - outside is closed, inside they're open.
    – Allan
    Commented Feb 15, 2018 at 12:07
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    Ok, so in this situation a security problem is still possible from INSIDE my home network, or if someone is able to enter into from external, I suppose. Now my problem is to understand what softwares are using these internal ports and the way to eventually close them
    – SILminore
    Commented Feb 15, 2018 at 12:17
  • Yes to both counts. If you decide to enable WAN management, you expose your router. The OS on the router is what uses those ports. You have the option to turn them on/off.
    – Allan
    Commented Feb 15, 2018 at 12:54

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