Does NAT occur before or after routing?

dellpe

Hello,everybody,I got another problem in here.
Recently we are deploying our network, I got my cisco CISCO2901/K9 router. Now, I get one problem about it, because we have two ISPs, so I wonder does nat occur before or after routing? I couldn't find any explain about this from the datasheet in cisco.com.

FloOz

Inside to outside translation occurs after routing, and outside to inside translation occurs before routing

NAT Order of Operation  [IP Addressing Services] - Cisco Systems

JaneDoe

You can only route a public IP address on the internet, so when data leaves a NATed network, address translation occurs before the data can be routed, when information enters a NATed network, address translations occurs after it arrives. Sometimes you can route traffic in a NATed network (many examples in the CCNA books show this) as long as you are routing between multiple internal destinations; however that traffic must be translated before it can go to the internet.

If you understand that no traffic with a private address can ever go on the internet, and NATing is the process of switching between public and private IP addresses, then it's easy to understand where NATing must occur. NATing is the main function of many home and small business routers.

veritas_libertas

NAT'ing is not just for Private-to-Internet translation. You can NAT between private networks as well. It's simply translation.

JaneDoe

You can Nat between private addresses, but I have no idea why anyone would do that outside of a lab. You can also NAT public addresses to other public addresses, and you many need to do that if your ISP gives you a different set of addresses than you previously had, and your old IP address is hardcoded in software somewhere.