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I'm quite new to networking, just learning the basics. I have a doubt, hope you can solve it.

Suppose I have a LAN ip 192.168.0.103 and I have a public IP of 66.249.79.122.

Now I have read that whenever my machine is communicating with another machine in the world wide web, there comes router that creates a map and keeps track of websites or machines I'm talking to so that replies to my public ip from those networks are directed to me.

Now my question is if there's another machine that wants to communicate with me using telnet of ftp then how can it? Because if it knows my public ip and sends a packet there the router will not know where to deliver that packet.

This is what I'm thinking. Can anyone please explain if I'm correct or not and if I'm then is it possible to deliver the packet in anyway?

Any help is appreciated. Thanks in advance.

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To enable someone from the WAN (Wide Area Network) to communicate to your PC in the LAN (Local Area Network) with telnet ftp ssh etc.., your router must use NAPT (Network Address Port Translation).

When a PC from WAN (Internet) will telnet your PC (LAN) he will actually telnet your public ip adress which is the router interface on the WAN side with a port number, by default it's 23 for telnet.

If you want someone from the WAN to telnet you in your LAN you need to setup the PAT on your router, it work like this, you configure your router to make him forward the packet he will receive on a specific port number to a computer on your LAN and to a port number of this computer.

Let's say someone from WAN telnet your public ip adress 66.249.79.122 He will probably use the default port number of 23, you configure your router to forward all the traffic from port 23, to your local ip 192.168.0.103 on the port 23, so when someone will telnet you he will actually telnet your router which will forward the request to your LAN PC.

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Internet hosted resources can access through internet by static nat that is one to one natting

As you described resource in internal Lan server can be accessed from internet on any required port

For this requirement static public IP is required which is provided by ISP . Further configured one to one natting (static natting)

Asumming internal server ip address 10.235.10 5/32 , public ip address 253.87 23.X

Assuming your are using cisco router as perimeter device .

Router(config)# IP NAT inside source static 10 235.10.5 253.87.23.X

With static public IP address 253.87.23.X on respective port internal hosted server can be accessed from internet .

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