Dynamic NAT maps the private IP addresses to the first available Public Address from a pool of possible Addresses. In the FortiGate firewall this can be done by using IP Pools.
This is a form of Dynamic NAT that maps multiple private IP address to a single Public IP address but differentiates them by using a different port assignment. This is probably the most widely used version of NAT. This is also referred to as PAT (Port Address Translation) or Masquerading.
An example would be if you had a single IP address assigned to you by your ISP but had 50 or 60 computers on your local network.
Say the internal address of the interface connected to the ISP was 2184.108.40.206 (again an impossible address) with 2220.127.116.11 being the remote gateway. If you are using this form of NAT any time one of your computers accesses the Internet it will be seen from the Internet as 218.104.22.168. If you wish to test this go to 2 different computers and verify that they each have a different private IP address then go to a site that tells you your IP address such as www.ipchicken.com. You will see that the site gives the same result of 222.214.171.124, if it existed, as the public address for both computers.
As mentioned before this is sometimes called Port Address Translation because network device uses TCP ports to determine which internal IP address is associated with each session through the network device. For example, if you have a network with internal addresses ranging from 192.168.1.1 to 192.168.1.255 and you have 5 computers all trying to connect to a web site which is normally listening on port 80 all of them will appear to the remote web site to have the IP address of 2126.96.36.199 but they will each have a different sending TCP port, with the port numbers being somewhere between 1 and 65 535, although the port numbers between 1 to 1024 are usually reserved or already in use. So it could be something like the following:
|192.168.1.10||2188.8.131.52: port 486|
|192.168.1.23||2184.108.40.206: port 2409|
|192.168.1.56||2220.127.116.11: port 53763|
|192.168.1.109||218.104.22.168: port 5548|
|192.168.1.201||222.214.171.124: port 4396|
And the remote web server would send the responding traffic back based on those port numbers so the network device would be able to sort through the incoming traffic and pass it on to the correct computer.
Because everybody is using the relative same small selection of Private IP addresses it is inevitable that there will be two networks that share the same network range that will need to talk with each other. This happens most often over Virtual Private Networks or when one organization ends up merging with another. This is a case where a private IP address may be translated into a different private IP address so there are no issues with conflict of addresses or confusion in terms of routing.
An example of this would be when you have a Main office that is using an IP range of 172.16.0.1 to
172.20.255.255 connecting through a VPN to a recently acquired branch office that is already running with an IP range of 172.17.1.1 to 172.17.255.255. Both of these ranges are perfectly valid but because the Branch office range is included in the Main Office range any time the system from the Main office try to connect to an address in the Branch Office the routing the system will not send the packet to the default gateway because according to the routing table the address is in its own subnet.
The plan here would be to NAT in both directions so that traffic from neither side of the firewall would be in conflict and they would be able to route the traffic. Everything coming from the Branch Office could be assigned an address in the 192.168.1.1 to 192.168.1.255 range and everything from the Main office going to the Branch Office could be assigned to an address in the 192.168.10.1 to 192.168.10.255 range.
In Static NAT one internal IP address is always mapped to the same public IP address.
In FortiGate firewall configurations this is most commonly done with the use of Virtual IP addressing.
An example would be if you had a small range of IP addresses assigned to you by your ISP and you wished to use one of those IP address exclusively for a particular server such as an email server.
Say the internal address of the Email server was 192.168.12.25 and the Public IP address from your assigned addresses range from 2126.96.36.199 to 2188.8.131.52. Many readers will notice that because one of the numbers NAT
is above 255 that this is not a real Public IP address. The Address that you have assigned to the interface connected to your ISP is 2184.108.40.206, with 2220.127.116.11 being the remote gateway. You wish to use the address of 218.104.22.168 exclusively for your email server.
When using a Virtual IP address you set the external IP address of 222.214.171.124 to map to 192.168.12.25. This means that any traffic being sent to the public address of 2126.96.36.199 will be directed to the internal computer at the address of 192.168.12.25
When using a Virtual IP address, this will have the added function that when ever traffic goes from 192.168.12.25 to the Internet it will appear to the recipient of that traffic at the other end as coming from 2188.8.131.52.
You should note that if you use Virtual IP addressing with the Port Forwarding enabled you do not get this reciprocal effect and must use IP pools to make sure that the outbound traffic uses the specified IP address.
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