NAT Extendable on Cisco IOS

Network Address Translation (NAT) is used to translate internal IPv4 addresses, which are typically private IPs to external IP addresses, typically public addresses. However, when there are two or more ISPs that serve a particular network, you may want to translate a private address to multiple public addresses.

This can be achieved using the NAT extendable feature. This feature is configured using the following commands with the extendable keyword:

R1(config)#ip nat inside source static extendable R1(config)#ip nat inside source static extendable

The above configuration allows the address to be translated to two different outside addresses.

If the extendable key word is not used, the IOS will respond with an error:

R1(config)#ip nat inside source static R1(config)#ip nat inside source static % already mapped ( ->

The purpose of the extendible keyword here is to allow multiple OUTSIDE to INSIDE NAT translations to take place to the same inside host address. This is useful because users on the Internet can be routed via either ISP to reach that host.

However, having the host reach the Internet via two different ISPs is not a problem that the extendible keyword is used to solve. If you want to achieve this, for redundancy for example, there are other ways to do it such as SD-WAN, Equal-cost Multi-path routing , or EtherChannel for example.