Rashmi Bhardwaj | Blog,Config & Troubleshoot

NAT Overload

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 

NAT Interview Questions


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 for WAN Connectivity to ISP
  • Additional Public IP of for customer access to the Internet.

In order for multiple LAN Users ( to access the Internet via Single Public IP i.e., 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 –

R1 Configuration

R1(config)#int fa0/0
R1(config-if)#ip addr
R1(config-if)#no sh
R1(config)#no ip routing
R1(config)#ip default-gateway

R2 Configuration
R2(config)#int fa0/0
R2(config-if)#ip addr
R2(config-if)#no sh
R2(config)#no ip routing
R1(config)#ip default-gateway

R3 Basic configuration –
R3(config)#int fa0/0
R3(config-if)#ip addr
R3(config-if)#no sh
R3(config)#int fa0/1
R3(config-if)#ip addr
R3(config-if)#no sh
R3(config)#ip route

Now , we will configure R3 to perform NAT Overload as below –
R3(config)#ip nat pool NATPOOL netmask
R3(config)#ip access-list standard LANPOOL permit
R3(config)#ip nat inside source list LANPOOL pool NATPOOL overloadR3(config)#int fa0/0
R3(config-if)#ip nat inside
R3(config)#int fa0/1
R3(config-if)#ip nat outside

cisco nat overload

Once the NAT Overload configuration is complete, we will verify the same –

On R1 (ping any Global IP – In this case lets say –

R1# ping
R1(config)#do ping
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 88/105/124 ms
Now , lets view the R3 NAT Transalation table –
R3#sh ip nat translation
Pro     Inside global      Inside local       Outside local      Outside global
The same ping is performed on R2 as below –
R3#sh ip nat translation
Pro    Inside global      Inside local       Outside local      Outside global
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 ( can also be used as Inside Global Address for NAT Overload instead of dedicated IP address (in this case
Some more useful NAT commands are –
clear ip nat translation *
show ip nat statistics



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