Hi Karthik,
These terms can be quite confusing–I am still baffled why Cisco continues to use them! This is what has helped me …
As you know, there are four possible varieties:
- Inside Local
- Inside Global
- Outside Local
- Outside Global
Notice the left word is always either “Inside” or “Outside.” Think of this as the origin of the packet with respect to a NAT. Did the packet originate inside or outside the NAT?
The word on the right is always either “Local” or “Global.” Think of this as where you have captured the packet you are looking at with respect to the NAT. Have you captured it within your own network so it would be local, or did you capture it beyond your own network so it would be global?
So, using this understanding, let’s talk about Outside Global and Inside Global.
Outside Global: We are talking about a packet that originated outside the NAT and you are viewing this packet NOT within your own network. This means that Outside Global address is IP address the packet is using, as assigned by someone else, while it traverses an external network. In Internet terms, this would be an globally unique, routable IP address.
Inside Global: We are talking about a packet that originated inside the NAT, but you happen to be looking at it from beyond your network. To understand what kind of address this might be, you would have to know whether your NAT is translating packets that come from your inside network onto an external network. If the NAT does NOT, then the Inside Global address is probably the same as the Inside Local address (this would rarely be the case). If you have a policy that changes the source address of packets as they leave network, then the Inside Global address will come from the address (or pool) you defined for that policy. In the case of using NAT to access the Internet, the Inside Global address would be a unique, routable IP address as assigned by your NAT.