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I agree with Mr. Jay Joshi in his answer.
NAT stands for Network Adress Translation.
NAT functionality does this translation task of assigning Public IPs to the Outgoing / Transmitted Packets and assigning of respective Private IPs to the Incoming / Received Packets.
NAT can be used for two reasons:
1- To add another layer of Security to the firewall configuration through hiding the real addressing of the inside private network.
2- If you don't have enough number of public IP Addresses to access the external network you use NAT with overload (PAT) to assign a larger number of client addresses to a lower number of Public IP adresses.
NAT stands for Network Adress Translation.
1. When a Machine is connected to Internet via a Firewall / Router, the Machine is assigned with a Private IP Address.
2. This Private IP Address is unknown to the Public Web and the Packets cannot be routed to Private IPs.
3. NAT functionality does this translation task of assigning Public IPs to the Outgoing / Transmitted Packets and assigning of respective Private IPs to the Incoming / Received Packets.
4. Whenever a Packet is transmitted from a Private Network to a Public Network, a Public IP / Internet IP is assigned to the the Packet and whenever a Packet is transmitted from a Public Network to a Private Network a Private IP is assigned to the Packet i.ie. its redirected to the respective Private IP.
As far as the question 'why NAT on firwall' is concerned as you probably know firwalls are often installed as entry / exit point of your network. So this is the point where you wish to utilize NAT dual function of translating your address from unregistered/private to public and keeping internal network IP configuration hidden from outside world for security.
Your question is "Why firewalls often have network address translation (NAT) functionality ?"
To be specific to your qestion I understant you know what is NAT so let me tell you here that Firewall is a security device and funtion of security devices such as firewall is to protect your network from outside world.The usual practice for using a firewall is to allow only necessary traffic and to block everything else.So what is special in it?
To add value to the money you spend on a branded firewall is because of its features as NAT,PAT,SNAT etc. You always look for more that is the reason why UTM based firewall have come which has all in it such as IDS. anti-virus etc. Technology needs to be advanced when you really want to protect your network from the bad outsite world.
