What is NAT?

NAT is an acronym for Network Address Translation. It removed the need for multiple, global IP addresses. If you intend to connect your current LAN to the Internet, it is likely you will use a form of NAT to overcome the need for each each computer to have a global unique IP address. Instead you can use private addressing in the ranges:

10.0.0.0 to 10.254.254.254

172.16.0.0 to 172.31.254.254

192.168.0.0 to 192.168.254.254

All traffic for the Internet goes through one external host - usually a router. 

NAT allows your router to be supplied with a single, real RIPE registered IP address already assigned to it. When your PC requests information from the Internet, the router converts the private IP address of the PC into the single real IP address of the router and then makes the request for information on behalf of the PC. The router keeps track of which internal PC requested which piece of information and passes the correct response from the Internet back to the correct PC. All PC's on your LAN will appear to the outside world as if they all have the same IP address. This will increase security as this IP address is not on any of the PC's themselves but on the router.  

If you need to use services that are either pushed at/sent to your network (e.g. SMTP mail) or information that needs to be requested from a network (e.g. Web pages on a HTTP server in your network) then you will require at least one static IP address (Typical installations use two IP addresses, one for the ADSL router, and one for a hardware Firewall).  You could still use NAT on your local area network in such instances, but the ADSL router would have to have a static IP address to able to be contacted via external computers wishing to use such services as SMTP on your network.  At time of ordering ADSL, you need to request a number of static IP addresses and a 'No NAT' setup ADSL router. You might consider then using a hardware Firewall to run NAT services for your local area network.