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So the private addressing scheme works really well for computers that only have to access resources

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inside the network, like workstations needing access to file servers and printers and so forth.

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Routers inside the private network and route traffic between private addresses with no trouble whatsoever,

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however, to access resources outside the network, like the Internet, for example, these computers

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need to have a public address so that responses to their requests return to them.

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Now, this is where Nat comes into play.

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A workstation inside a private network makes a request to a computer on the Internet.

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The switches or the routers within the network recognize that the request is not for a resource inside

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the network, so they send the request to the router, let's say the backbone router.

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Now, the backbone router sees the request from the computer with the internal IP.

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It then makes the same request to the Internet using its own public address and returns a response from

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the Internet resource to the computer inside the private network.

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From the perspective of the resource on the Internet, it's sending information to the address of the

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router.

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From the perspective of the workstation, it appears that the communication is directly with the site

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on the Internet.

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So when that is used like this, all users inside the private network that access the Internet have

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the same public IP address.

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So that means only one public address is needed for hundreds or even thousands of users.

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So let's have a look and see how it works.

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Here's a typical home network.

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There's a laptop, desktop, PC, smartphone, all connected to our home router and a home router is,

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of course, connected to the Internet.

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So when we register with our ISP, the Internet service provider, we get an IP address that is accessible

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throughout the Internet and that's pretty much assigned to that home router.

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We refer to it as real IP or public IP.

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Now suppose that the IP address two zero two zero two zero to zero is assigned to our home router.

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Our devices inside the home network also have IP addresses, right, but in this case, they get private

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IP addresses which are assigned by the home router and are not accessible from the Internet.

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So as you can see, here are the public IP addresses are red and the private IP addresses are green.

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So let's consider a connection request from the smartphone, which request the home page of ABC Dotcom.

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So to reach the page, the smartphone has to go through the home router.

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The packet has the source IP address and the source port address, as well as the destination IP address

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and the destination port.

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If it arrives at the Web server with these values, it processes the request and tries to send the reply

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packet to the IP address one nine two dot one six eight one five.

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But that's unreachable for the Web server because it's a private IP address.

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So when the packet arrives at the home router, instead of sending the packet right over the Internet,

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the home router changes this source IP address with its very own public IP address.

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It also creates a record in the net forwarding table.

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This table allows us to know which packets will be redirected to the smartphone when they come in.

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So moving right along, the packet travels over the Internet and arrives at the Web server.

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The Web server creates a reply packet where the source IP address is itself and the destination IP address

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is the public IP address of the home router.

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When our home router receives a response, it looks at the net forwarding table and replaces the destination

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IP and the port, according to the mapping inside the table.

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And finally the smartphone receives the packet.

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Pretty cool, huh?