Network Address Translation (NAT)
NAT is a technology that allows a private network to share an Internet connection using a single IP address. All clients (computers, mail servers etc) behind the router or firewall are hidden behind the IP address allocated to that device. This means that in many circumstances only a SINGLE fixed IP address is required. As an added bonus, this option provides a higher level of security as client machines are effectively hidden behind the NAT device and cannot be contacted directly from the Internet.
Port Address Translation (PAT)
In some situations there may be a requirement for certain clients hidden behind the NAT device to have direct contact from the external Internet (eg. a web server or mail server). The solution is to set up PAT (also known as port forwarding) which re-directs requests from certain ports on the router to a specific internal client.
See below some examples of situations where NAT is a more suitable option than multiple IP address allocation. In these situations we will NOT BE PERMITTED BY RIPE to allocate multiple addresses for your account:
REQUEST: I have X number of clients I want to connect to the Internet so I need X IP addresses
ANSWER: You should use NAT on the router so that all clients share a single IP address
REQUEST: I wish to run a web server and a mail server at the end of my ADSL connection
ANSWER: You should use NAT but also set up PAT to direct mail and web traffic to your internal servers
REQUEST: I want to set up multiple web servers for testing purposes
ANSWER: You should run your web servers on different ports and set up PAT to direct the traffic to the appropriate internal web server
REQUEST: I wish to use ICQ (chat) on one or more client machines
ANSWER: Set up ICQ to listen on different ports for each client and set up PAT to direct the ports to each.
REQUEST: I want to use Securemote to connect to an external network
ANSWER: Set up Securemote to use IKE
REQUEST: I need to have my firewall running behind my router and would like a block of 5 usable IP addresses
ANSWER: We can allocate a block of 2 addresses, one for the router and one for the firewall
REQUEST: I want to connect to service X on my internal machines remotely
ANSWER: In many instances it is possible to run the services on different ports on each machine and use PAT on each of the ports
REQUEST: I want to run 2 DNS servers at the end of my ADSL connection
ANSWER: An ADSL connection is not recommended for this purpose. Primary and Secondary DNS servers should also be located on separate networks for resilience.
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