ip nat inside source list [acl] interface [intf] overload

Overview

The `ip nat inside source list [acl] interface [intf] overload` command is a cornerstone of Network Address Translation (NAT) configuration on Cisco IOS devices. It enables Port Address Translation (PAT), also known as NAT overload, which allows multiple private IP addresses from an inside network to share a single public IP address assigned to a specific interface. This is critical for conserving public IPv4 addresses, as it maps many inside hosts to one outside address by differentiating traffic based on source port numbers.

The command works in conjunction with an access control list (ACL) that defines which inside source addresses are eligible for translation. When a packet from an inside host matches the ACL, the router translates the source IP address to the IP address of the specified interface (typically the outside interface) and assigns a unique port number. The translation is dynamic: entries are created when traffic initiates and are removed after a period of inactivity.

This command is typically used in scenarios where an organization has a limited number of public IP addresses (often just one) and needs to provide internet access to many internal users. It is an alternative to static NAT (which maps a specific inside address to a specific outside address) or dynamic NAT without overload (which requires a pool of public addresses). PAT is the most common form of NAT in home and small-to-medium business networks.

In a broader configuration workflow, you would first define the inside and outside interfaces using `ip nat inside` and `ip nat outside`, then create an ACL to match the private addresses, and finally apply this command. The command is executed in global configuration mode and requires privileged EXEC access (enable mode). The running configuration will show the NAT configuration, and the translations can be verified with `show ip nat translations`.

A common pitfall is forgetting to apply the `ip nat inside` and `ip nat outside` commands to the respective interfaces, which will cause NAT to not function. Also, the ACL must be properly defined; a common mistake is using a standard ACL that does not include the correct source addresses or using an extended ACL unnecessarily. The command is supported in IOS 12.x and later, including IOS-XE, with consistent syntax.

It is not available in NX-OS or IOS-XR, which use different NAT configuration models. Understanding this command is essential for CCNA and CCNP candidates as it appears frequently in exam scenarios and real-world network design.