As you probably know, a router’s primary purpose is to regulate traffic flow between two networks, and a NAT router is no exception. The server that you will use as a NAT router must have two network interface cards (NICs) installed. One of these NICs will connect to the Internet and the other will connect to the private network. PCs on the private network will then send HTTP requests to the NAT server via the server’s private network connection. The server will then retransmit the request over the Internet on behalf of the client. When the requested Web site responds, the response is sent to the NAT server, which in turn forwards it to the client who made the original request. The client never communicates across the Internet directly.
IP Addressing Considerations
As I explained in the section above, a NAT router acts as a gateway between your private network and the Internet. The server that is acting as the NAT router must have two NICs. One of the NICs is connected to the Internet. This NIC must be assigned the IP address that was given to you by your Internet Service Provider.
The other NIC connects to your private network. As I mentioned, NAT does not expect you to have valid IP addresses on your private network. Instead, you are basically free to pick an address range at random. There is the off chance that the range that you pick might already be in use by a popular Web site, but I have only seen someone pick an address range that caused problems once. If you want to use an address range that is guaranteed not to interfere with anything on the Internet, you can use the 192.168.x.x address range.
After you pick an address range, I recommend setting up a DHCP server so that it will assign addresses from your chosen address range (the DHCP term for an address range is a scope) to the workstations on your network. You must however statically assign an address to the NIC on the NAT server that connects to your private network. For example, if you chose to use the address range 192.168.1.0 to 192.168.1.99, then you might consider assigning the address 192.168.1.0 to the NAT server. You could then use the 192.168.1.1 to 192.168.1.99 address block as your DHCP scope.
While you are configuring your DHCP server, there are a couple of other considerations that you need to make. As you may know, DHCP allows you to optionally assign a default gateway and a DNS server to workstations along with an IP address. When doing so, you must set the default gateway address to match the private network address that you assigned to your NAT server.
You have a few different options when choosing which DNS server address the DHCP server should assign to the workstations on your network. If you don’t have your own DNS server, then the best thing that you can do is to just use the IP address of your Internet service provider’s DNS server. If your network is running Active Directory though, then you already have a DNS server and you should use its address. It doesn’t matter if your DNS server is authoritative for your domain or not. Simply point the workstations to it. You can then set up a forwarder on the DNS Server so that any unresolved queries get forwarded to your ISP’s DNS server.
The advantage to pointing clients to your own DNS server rather than to your ISP’s DNS server is that doing so will provide your users with better performance. Your DNS server is local, so queries reach the server more quickly than they would reach a remote server. Furthermore, your DNS server has a built in cache so that popular Web sites do not have to be resolved each time a user visits them.
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