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I am new to the world of network so please bear with me. I have chained two routers to create two subnet at my home. I have two routers and second router is connected to my ISP ADSL modem router by wire on WAN port (LAN to WAN). So the topology is loosely like this -

Internet ------ ADSL router 1 (10.0.0.138) ----- router 2 (192.168.1.1)

I have configured the EWAN settings on router 2 and set a static external/public IP for router 2 (10.0.0.160)

I can ping the devices of network 10.0.0.x from 192.168.1.x network but not vice versa. I can't understand why? Is it something not possible or I am doing something wrong or may be a firewall at router blocking the communication. Please advise what I am missing something here.

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  • Home networking and consumer-grade device are off-topic here. You might want to try on Super User.
    – Zac67
    Aug 16 '18 at 6:13
  • @Zac67 Because the reason of the problem described is a "technological" problem I will answer anyway because the answer will explain network technology theory... Aug 16 '18 at 6:16
  • Well, the basic reason of the OP's problem is the inherent NAT routing of consumer routers.
    – Zac67
    Aug 16 '18 at 6:22
  • @Zac67 Yes. That's why I'm writing what NAT is... Aug 16 '18 at 6:22
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I am new to the world of network ...

So I will simplify a lot and explain the technical details incorrectly to make my answer more understandable.

I can ping the devices of network 10.0.0.x from 192.168.1.x network but not vice versa.

Technical background

Obviously you don't have "real" subnets but you use so-called "NAT".

NAT routers do not "really" route IP packets but they simulate a different network topology to the other computers in the network.

A typical NAT router has two sides: The local network (in your case: 192.168.1.x) and the internet (in your case: 10.x.x.x).

To the computers in the local network the NAT router "simulates" a network topology where the "local" computers are "directly" connected to the internet. In other words: It "simulates" a network topology where 10.1.2.3 can directly send data to the address 192.168.1.123.

To the computer in the internet the NAT router "simulates" that the whole network 192.168.1.x is one single computer: 10.0.0.160 in your case.

So if 192.168.1.123 establishes a connection to 10.1.2.3 the NAT router "simulates" a network in a way that 10.1.2.3 will "think" that there is a connection to (from) 10.0.0.160, not to 192.168.1.123.

This was invented because nearly all internet service providers support only one computer to be connected to an internet connection. Using NAT routers you are able to connect multiple computers to the internet connection while the router simulates that only one computer is connected.

Your Problem

You can of course establish connections from 192.168.1.123 to 10.1.2.3:

As described before 10.1.2.3 will "think" that the connection was established from 10.0.0.160, not from 192.168.1.123.

But establishing a connection to 192.168.1.123 won't work:

First 10.1.2.3 does not know about 192.168.1.123; it "thinks" the address of this computer is 10.0.0.160.

Second you could add a third NAT router to your system: 10.0.0.161.

You could connect another computer with the address 192.168.1.123 to that router so there are two computers having the same address in your whole network.

To which of both computers shall a connection be established?

For this reason NAT routers won't support this.

You could also try to establish a connection to 10.0.0.160 because 10.1.2.3 thinks that 192.168.1.123 has the address 10.0.0.160.

Then you'll have the problem that the NAT router does not know if the connection shall be established to 192.168.1.123 or to 192.168.1.145.

However many NAT routers can be configured that certain types of connections (for example HTTP) from the internet (10.1.2.3) to the router (10.0.0.160) will actually establish a connection to a certain computer behind the router (e.g. 192.168.1.123). This feature is typically called "port forwarding".

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  • Thanks for explanation. It was really helpful. You said that I don't have real subnet. Is it because my router doesn't support it? I saw this video on youtube and followed it - youtube.com/watch?v=dIFKmJ4wufc
    – akajain
    Aug 16 '18 at 11:14
  • Why someone has down voted this answer? Please explain.
    – akajain
    Aug 19 '18 at 2:39

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