I'm relatively new to the network world and a question came into my mind: Why can I nmap a network behing a router and see ports 80/443 open but I can't access it from outside? Is this some kind of rule built into the router itself that only allows local networks to access it?

I'm talking about any router, like TL-WR841ND for example.

  • You should provide a lot more details, unless you want just guesses. – RalfFriedl Oct 26 '18 at 16:43
  • I don't have more details. All I know is, if you try to connect to externalip it will fail (but nmap still shows port 80/443 open), while acessing internalip (if inside network, of course), it works. – Asdra Oct 26 '18 at 16:47

Unless you have a proxy configured on the network and/or host an IDS (HIDS) or Network (NIDS) “Intrusion detection system” There is no way to analyze these two ports from outside the network. 443 is HTTPS which is encrypted traffic and 80 is the World Wide Web and both are always open. You cannot do a port scan from outside your network and see such ports and their activity due to “Port knocking” which requires a specific “knock sequence”. Without this security protocol, anyone can scan your internal network from outside your network and find every exploit on the network. Nmap can only analyze/communicate with “Target Hosts” thus the need for a Proxy or IDS host on the network... Once you set this up, you can use nmap to properly scan the network for possible exploits and vulnerabilities.

I hope this helps!

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