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NAT, Network Address Translation in simple terms translates an IP address into another. Network Address Translation is of different types like

Static NAT (One to One)

Dynamic NAT (Many to Many)

Overloading (Many to One)

We will focus on Overloading form of NAT. This is called as Port Address Translation (PAT) or even Network Address Port Translation (NAPT). NAT Overloading translates many private IP addresses from a Local Area Network (LAN) onto a single registered legal Public IP address. Here, the source IP and the source port gets translated to the Public IP and a different source port.

Typical network configuration would be on an Internet Router which enables all the hosts in the LAN to connect to the Internet using one single Public IP address.

The following procedure will help you to configure NAT Overload or Port Address Translation (PAT) in Cisco IOS:

1. NAT inside interface

Enable an interface on the router with an IP Address and mark it as nat inside interface. This is the interface that connects to your internal private network

WANRouter(config)# int fastethernet0/1

WANRouter(config-if)# ip address 192.168.1.1 255.255.255.0

WANRouter(config-if)# ip nat inside

2. Enable NAT outside interface

WANRouter(config)# int serial0/0/0

WANRouter(config-if)# ip address 100.100.100.100 255.255.255.0

WANRouter(config-if)# ip nat outside

3. Configure NAT Pool

This will be a pool of legal Public IPs that is bought by the organisation. This could anything from one to many IP Address

WANRouter(config)# ip nat pool WANPOOL 100.100.100.10 100.100.100.10 netmask 255.255.255.0

This creates pool which has just one IP address. The syntax is

ip nat pool <pool name> startip endip {netmask netmask | prefix prefix-length}

4. Access List to allow list of IP Addresses to NAT translate

WANRouter(config)# ip access-list 10 permit 192.168.1.0 0.0.0.255

For more networks or hosts to overload the NAT pool simply add them to the access list

WANRouter(config)# ip access-list 10 permit 192.168.2.0 0.0.0.255
WANRouter(config)# ip access-list 10 permit 192.168.3.0 0.0.0.255

Instruct Router to NAT the Access list to the NATPool

WANRouter(config)# ip nat inside source list 10 pool WANPOOL overload

If this is an internet configuration then ensure that a default route on the IP to the outside IP address or outside interface

WANRouter(config)# ip route 0.0.0.0. 0.0.0.0 serial0/0/0

or

WANRouter(config)# ip route 0.0.0.0 0.0.0.0 100.100.100.100

Thats it. Job done!!!

The NAT setup is now complete.We have setup the router to translate LAN private IPs into the Internet public IPs.

To check the NAT status and statistics

WANRouter# show ip nat statistics

To see the active translations

WANRouter# show ip nat translations

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