In this video, we will learn Inside Local Address, Inside Global Address, Outside Local Address and Outside Global Address in Network Address Translation in Cisco Routers.
0:00 Intro
0:09 Cisco NAT Terms
0:59 Inside VS Outside
1:49 Local VS Global
2:27 NAT Topology
2:48 Inside Local Address
3:15 Inside Global Address
4:10 Outside Global Address
4:37 Outside Local Address
7:29 Visit Us
NAT Course Link
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doctornetworks.net/courses/ci...
Let's start by understanding the terms INSIDE & OUTSIDE that are used here
INSIDE vs OUTSIDE
Generally speaking, the INSIDE & the OUTSIDE describe the "Physical location" of a device.
That device could be a desktop, laptop, Server, mobile phone, IP camera or any such device that works with an IP address.
Now the "INSIDE" term refers to the “devices” that are residing inside of your organization physically speaking.
For example your organization’s workstations & servers that reside behind your NAT boundary router will be considered “INSIDE” of your organization.
The term "OUTSIDE" refers to the device's residing outside of your organization, again physically speaking. For example, Google or Facebook servers would be considered “outside” of your Organization.
LOCAL vs GLOBAL
Now let's look at the terms LOCAL & GLOBAL.
Generally speaking, the terms “LOCAL” & “GLOBAL” refer to the PRIVATE & PUBLIC IP addresses respectively, but with Cisco’s NAT terms they refer to the “viewpoint” or the “perspective” of the device when it looks at an IP address.
For example, A LOCAL device sitting inside of your organization has its own perspective or viewpoint of looking at an IP address.
A device outside of your organization view’s an IP addresses with their own perspective or viewpoint. Now I know that is confusing, but bear with me it will be cleared in the upcoming slides.
INSIDE LOCAL ADDRESS
Now these terms that we refer to are displayed on the cisco NAT boundary router as an IP packet traverses it. You can view it with the “Show IP NAT translations “command on a Cisco router.
Let's say PC-A has a web browser open & is connecting to the website, www.doctornetworks.net via HTTP.
Now an “inside local address” is the local IP address of a private host on your network, in this case it’s PC-A.
Now PC-A resides inside of your organization physically speaking so it justifies the INSIDE part & LOCAL is the perspective, so from the perspective of the internal or local devices, they see that address as 192.168.1.10.
INSIDE GLOBAL ADDRESS
Now the INSIDE GLOBAL ADDRESS is the PUBLIC or REGISTERED IP address that the outside networks see as the IP address of your local host ,PC-A in this case.
Now a quick question here, where does the device or PC-A physically exist even though it’s IP has been translated to a Public IP address?
It still physically resides inside your organization right? So that justifies the INSIDE part.
Now Global is the perspective, So from the perspective of all the users on the internet, PC-A will be seen as its GLOBAL or PUBLIC IP address. Meaning when PC-A accesses doctornetworks.net, its IP will be seen as 200.1.1.1 in this case.
OUTSIDE GLOBAL ADDRESS
I will leave OUTSIDE LOCAL ADDRESS for last as it’s a weird one.
The OUTSIDE GLOBAL ADDRESS is the PUBLIC IP address of the remote host, for example the PUBLIC IP address of the remote Server that a workstation is connecting to.
For our example, it will be the PUBLIC IP of the WEB Server hosting doctornetworks.net.
Now It is indeed physically outside of your organization & from the GLOBAL perspective or viewpoint, they see that IP as 111.1.1.1
OUTSIDE LOCAL ADDRESS
Now here comes the weird one, OUTSIDE LOCAL ADDRESS is the local IP address from the private network which your local host sees as the IP address of the remote host.
Now if that isn’t frying your mind right now, nothing will! But don’t worry we got this, Lets just break down the terms & reverse engineer it.
Now, OUTSIDE signifies it’s a device sitting physically outside your organization. So it’s somewhere out there on the internet right?
LOCAL means the perspective of the LOCAL clients of how they see that IP address, In our case PC-A & PC-B will see that address as 111.1.1.1.
Now the confusion mainly arises here because we already have a term “OUTSIDE GLOBAL ADDRESS” that basically shows the same thing right?
I mean Think about it, if everyone else on the internet sees a PUBLIC IP as 111.1.1.1, aren't we gonna see the same thing as well? The answer to that is a resounding YES.
And that is the reason if you look at Cisco’s “Show IP nat translations' ' command Both OUTSIDE LOCAL & OUTSIDE GLOBAL have the same IP’s & port numbers.
So why the OUTSIDE LOCAL even present if the viewpoint is the same?
Well it is there to facilitate a specific type of NAT called “Destination NAT”
This is a type of NAT that has a very specific use & is very uncommon