In corporate networks or internet works on dominantly on IP protocol. IP protocols have two versions. Most popular default one is IPv4 and other next generation by not so widespread version is IPv6. IPv6 have a lot of advantages against IPv4 but as you know changing and infrastructure through world wide is not so easy job. Now our question is what is Network Address Translation simply NAT.
In Local Area Networks (LAN) IP range of 192.168.0.0/24 , 172.16.0.0/20 and 10.0.0.0/8 are used. These IP ranges are dedicated from IANA and do not used in internet. Other ip addresses are used by Internet. So the problem is what will happen when a host in the LAN want to access to the internet because LAN IP range cannot be used for internet? There must be an IP address used and reserved for internet. The address will be used for internet related network traffic. We will continue with the following IP address.
Host LAN IP Address: 192.168.1.10
Source Internet Address: 184.108.40.206
Destination Internet Address: 220.127.116.11
And our topology is like below;
To access destination ip address 18.104.22.168 LAN host 192.168.1.10 goes though firewall 22.214.171.124 . The ip address 126.96.36.199 is used by 192.168.1.10 in the internet to identify itself. Packets arriving to the 188.8.131.52 have source address 184.108.40.206 . We call this source address translation operation as NAT simply. Actually there are different type of mechanisms in NAT.