Networks, subnets, routing and NAT

Question

What I'm going to ask may be trivial but I really need to understand cause my teacher is confusing me!

That's the problem. I got 2 private networks

• The first is a class C network and the address is 192.168.1.0/24 (I'll call this C1)

• The second is a class A network which I divided in two subnets: 10.1.0.0/16 (A1) and 10.2.0.0/16 (A2)

Now I got one router between A1 and A2, and another between A2 and C1. I've configured the routing so that packets can go from A1 to A2 and from A2 to C1. Then teacher told me: "Router between A1 and A2 is ok, but A2 and C1 can't communicate cause they are different networks! You need to set up NAT in that router."

And here is the question: why do I have to use NAT here? I can't figure out why NAT is needed between two private networks!

Thanks for your help

Answer 1

To start, your teacher is wrong. On a side note: S/He should be reprimanded for even teaching Classed Networking, that was antiquated 16 years ago. It should not be taught, period (except in History class).

You do not need NAT in this case at all. Your three networks do not interface with the public Internet, and they are all using internal IP ranges.

NAT would be necessary if your networks were communicating with the Internet because your Internal IP ranges will not work on the Internet. Only non-Internal IPs work on the Internet. The NAT device takes a single (usually) Internet IP and allows the computers with Internal IPs to "fake" it (communications to the Internet will use the valid IP, and the NAT device keeps everything straight).

Answer 2

I think you need to use NAT just if want to access from private network to public one,

and you can use it to access resources of a private network from other private one.

Answer 3

Hai synasius,

I would like to suggest you to go through the following urls which may give a clear idea why your teacher had told like that?

http://192-168-1-2.org/how-nat-works.html

http://technet.microsoft.com/en-us/library/cc756722%28WS.10%29.aspx

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094831.shtml

Answer 4

Prograministrator was mostly right in his response, but let me be a bit more verbose...

Private addresses like 192.168.x.x and 10.x.x.x are only meant to be used within a local area network, not anywhere else. This is because they're highly overloaded. Millions of computers right now are simultaneously using the address 192.168.1.100. But that's okay, since NAT lets them do that. They only use that address when talking inside their own LAN. But when you talk outside the LAN, you have to use a public address.

While, strictly speaking, it's POSSIBLE to have two different network expose their internal addresses to one another and communicate that way, it's a very bad way to set it all up. Always assume that private IP's should not be seen anywhere outside their network.

What you should do instead is attach all the hosts in a network to a router and have them all NAT to one single public IP address.