BGP Private and Public AS Range

This topic is to discuss the following lesson:

Excellent! Thank you Rene!

“Removing the private AS numbers is a bit similar to NAT where we hide private IP addresses behind one or more public IP addresses”
But how are we going to do the mapping from private AS to public AS and back when the private AS number is not advertised by AS2 to AS 3 ?

Hi Nikhil,

We don’t. The only thing we do is remove the private AS number and then advertise the prefix(es). Take a look here:

BGP Remove Private AS

There’s no need to create a mapping between the private/public AS number.


Hey Rene,

In your last diagram in this lesson, suppose we have many private AS behind R2 which needs to go to the Internet, in this case how will R2 handle those sessions ? We all know that in similar situation in IPv4 world we have PAT which maps the private IP with one Public IP using unique port number how does R2 handle this situation? Appreciate if you shed some light on this.



Hi Sahil,

You can see it in this example:

BGP Remove Private AS

R2 will have the private AS paths in its own BGP table so it knows what to do.