The NAT extendable feature on Cisco IOS is used to map one private IP address to more than one Public IP address.Let us understand the use case of NAT extendable feature with below topology –
In this topology the ISP 1 and ISP 2 routers will try to connect to server on its IP 184.108.40.206.The IP address subnet for ISP 1 would be 220.127.116.11/24 and for ISP 2 would be 18.104.22.168/24.
Let us assume the IP assigned by ISP 1 is 22.214.171.124 and IP assigned by ISP 2 is 126.96.36.199.
We will configure a one-one NAT on R1 as below to make the Server reachable from ISP 1 first.
# A one-one NAT translation between 188.8.131.52 and 184.108.40.206.
Now if we want to configure the same thing for ISP 2 assigned public IP as below we get the error response from Cisco IOS:
To remedy this we can use the NAT extendable feature so the Server can be accessed from both the ISPs.
We see 220.127.116.11 is mapped to both the public IP (Assumed in out lab) 18.104.22.168 and 22.214.171.124. Related- NAT Types
We see 126.96.36.199 is mapped to both the public IP (Assumed in out lab) 188.8.131.52 and 184.108.40.206.
Related- NAT Types