NAT Overloading also called Port Address Translation (PAT) is a form of dynamic NAT where we have is just a single inside global IP address providing Internet access to all inside hosts. As a general case, cisco NAT Overload is used in scenarios where the number of inside local addresses is greater than the number of inside global addresses.
Related – NAT Types – Static, Dynamic And Overload
A simple scenario of cisco NAT Overload configuration will help the audience have a better understanding of Network address Translation concept and traffic flow across network elements.
In the setup, R1 and R2 routers in LAN have been configured as end systems (host machines) which are connected through a Layer 2 Switch (SW) to customer Gateway Router (R3).
The Gateway Router is further connected to Internet Service provider (ISP). The customer has been assigned Public IP address by ISP as below –
- Set of Public IP 126.96.36.199/30 for WAN Connectivity to ISP
- Additional Public IP of 100.100.100.1 for customer access to the Internet.
In order for multiple LAN Users (192.168.123.0/24) to access the Internet via Single Public IP i.e. 100.100.100.1, NAT feature of “NAT Overload” will be used here. NAT Overload, also known as PAT (Port Address Translation) is essentially NAT with the added feature of TCP/UDP ports translation.
The configuration for each device is shown below –
R3 Basic configuration –
Now , we will configure R3 to perform NAT Overload as below –
Once the NAT Overload configuration is complete, we will verify the same –
On R1 (ping any Global IP – In this case lets say 188.8.131.52) –
The same ping is performed on R2 as below –
Because these entries are all dynamically created, they are temporary and will be removed from the translation table after some time. Note – Router WAN Interface (184.108.40.206) can also be used as Inside Global Address for NAT Overload instead of dedicated IP address (in this case 100.100.100.1).
Some more useful NAT commands are –