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Each computer is assigned one ip address on my network, and my ISP supplies one IP for my network. If I try to scan the ip supplied by my ISP what I got is the OS running on the router, running on the firmware.

How do I scan my internal network from outside?

For example

Computers on my network:

192.168.0.200

192.168.0.201

192.168.0.202

192.168.0.203

IP supplied by my ISP 69.160.50.5

What TCP ports are open and which OS is running on 192.168.0.202? How can achieve this If all I have is 69.160.50.5? Thanks in advance.

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2 Answers 2

up vote 14 down vote accepted

First of all you need to understand how things actually work, so take a look here: NAT. This is how your router currently makes internet communication for all your hosts possible.

So, everything is behind a NAT, and that's exactly why you can't access any host using your public IP address. If you want a machine inside your private home network to be publicly available (I mean its services) you would have to give it access to it. In order to do that you would have either to forward some ports from your router to a specific machine inside your network or add the whole machine to the DMZ.

Example 1: I run a webserver at 192.168.1.30:80, so in order to make it accessible from the outside (using my public IP) I need to add a forwarding rule into my router configuration, so that port 80 would be "redirected" to 192.168.1.30:80. So if someone was to browse to http://my_public_ip:80 (you don't have to actually type ":80" here), his request would arrive to my router and it would be forwarded to my 192.168.1.30 machine, eventually showing whatever web page I was running on it. The same applies to every other service you want to make accessible to the public, let's say a FTP server. You can forward specific ports or ranges of them. Sample image:

port forwarding example

Example 2: As you realise the above scenario is not always practical. Sometimes you just want to make an entire machine/internal IP the default receiver of all outside requests. This is where the DMZ comes in. If I want every outside request to bypass my router and always arrive to a specific host I would add its IP (eg 192.168.1.123) to my DMZ. Sample image:

DMZ example

Take a look at this question too, for more insights concerning the use of DMZ. Don't be afraid to use it for your purposes though.

As you realise both of the above configurations are done inside your router interface.

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Thanks for the answer, Ion. –  Daniel Jan 17 '12 at 23:58

Ion has it right, but let me simplify it: you are scanning from the outside. The router's job is to prevent exactly what you are trying to do, that is, access the computers directly from outside.

To perform your scan, you either need to do it from the inside, or make the internal computers accessible from the outside. Either way, you will need access to the inside. Are we assuming you don't?

The 3rd option is to punch through the router, but that's a totally different topic.

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Thanks for summing up, schroeder. –  Daniel Jan 17 '12 at 23:57

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