NAT - Virtual Interface (NVI)
Network Address Translation (NAT) is a method used in IP networks (particularly in IPv4) to manipulate network address information in packet headers while in transit. It is commonly used to map private IPv4 addresses to a single public IP address or a few public IP addresses. This is essential for conserving public IP addresses and enabling internal network devices to access external networks like the Internet.
In the context of Cisco IOS routers, the NAT Virtual Interface (NVI) is a feature that simplifies the configuration of NAT. Traditionally, NAT is configured using inside and outside interfaces, where you have to designate which interfaces are 'inside' (private network side) and which are 'outside' (public network side).
With the NVI feature, you don't have to specify inside or outside interfaces. Instead, you use a single 'nvi' interface for NAT translations. Traffic that needs to be NAT'd is directed to the NVI interface, and the router takes care of the translations based on the NVI NAT configuration.
This simplifies the configuration and makes it easier to manage, especially in more complex network scenarios with multiple interfaces and VPN connections. It resolves issues involved with legacy NAT due to the predetermined order of operation, as well as scenarios where cumbersome methods are used to resolve such problems. It can also be useful in networks with dynamic routing because it removes the need to classify interfaces as inside or outside.