Network address translation (NAT) is a method of remapping one IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device(wiki).

In simple word, translating IP address to another IP is called NAT. Imagine that, we have one static IP address and we want to use this IP for diffenet services in local network. For example:


HTTP : 192.168.20. 101


Static IP:

How we can use this static IP address for different services?

NAT. By using NAT, you can map static IP address to different services, like below: 21 –> 80 –> 443 –>

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We can simple write in router to map IP to on port 21:

router# conf t
router(config)# ip nat inside source static tcp 21 21

If we have bunch of static IP address, we can define pool and access-list and pass it to NAT:

router# conf t
router(config)# ip nat pool mypool netmask
router(config)# access-list 1 permit
router(config)# ip nat inside source 1 pool mypool overload

“overload” simply translate all ports in static IP address to inside local address.