How to configure PAT on Cisco IOS Router

I have covered the configuration of static NAT and dynamic NAT in previous lessons, now it’s time for PAT. This is the topology we’ll use:

Let’s prepare the hosts. I am using normal Cisco routers with “ip routing” disabled to turn them into dumb hosts:

Host1(config)#no ip routing
Host1(config)#default gateway 192.168.123.3

Host2(config)#no ip routing
Host2(config)#ip default-gateway 192.168.123.3

Next step is to configure NAT:

NAT(config)#interface fastEthernet 0/0
NAT(config-if)#ip nat inside 

NAT(config)#interface fastEthernet 1/0
NAT(config-if)#ip nat outside

So far so good, let’s create an access-list that matches both hosts:

NAT(config)#access-list 1 permit 192.168.123.0 0.0.0.255

And finally we’ll configure PAT:

NAT(config)#ip nat inside source list 1 interface fastEthernet 1/0 overload

I select access-list 1 as my inside source and I will translate them to the IP address on FastEthernet 1/0. The big magic keyword here is overload. If you add this we will enable PAT!

Let’s give it a test run shall we?

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