In the section, we're going to discuss network address translation or NAT and Port Address translation
or PAT, we'll discuss why next is required.
Why do we use Nat in networks today?
We'll also discuss the difference between NAT and PAT and we'll discuss the Cisco term overloading,
which implies NAT, a lot of network engineers talk about NAT even though they actually mean PAT.
So we need to understand the differences between those two terms.
And as always, I'd like to demonstrate how to set up NAT using the command line interface.
We'll look at different types of NAT, including static NAT, dynamic NAT and PAT, or overloading, as Cisco
call it.
Before going any further.
Here's s a demonstration, I've disabled all network interface cards on my PC.
Except for the wireless interface card, when I look at the details of that card, I can see that my
IP address is 192.168.189
and I'll have a default gateway of 192.168.254.
I can view that in a CMD prompt as an example, so on my PC
type IP config, see the IP address, see the default gateway.
I can ping Google.com.
So notice I'm getting replies from Google.com.
I can access Google.com, so here's the Google website, but when I type what is my IP address into
Google.
I'm told by Google that this is my public IP address.
So
IP config again shows me that my internal IP address or private IP address is 192.1
68.189
but my public IP address is this.
So in this example, my Internet router at home is connected to the Internet and this router is configured
with DHCP, it's allocating IP addresses to devices internally using DHCP
and one of those IP addresses was allocated to my local PC
but when I go onto the Internet, my address used is a public IP address.
So network address translation.
essentially translates a private IP address, which is non-routable on the Internet to a publicly
routable IP address, your public IP address is typically allocated to you by your service provider,
which in my case is British Telecom or BT here in the U.K. In larger networks, companies may register
their own public IP addresses.
Here are some well-known examples.
Hewlett Packard owned 15.0.0.0/8 and 16, Apple own 17, MIT 18, Ford 19
AT&T 32.
and so on, and IBM, as an example, 9.0.0.0.
So as an example, if you're working within HP or within Apple, you may not be using a private IP address
such as this, you may actually have a public IP address allocated to your PC.
These are globally routable public IP addresses that were registered by some of the largest companies
in the world.
But IP addresses are running out.
So in most small medium businesses today, you'll be using a private IP address internally.