Global pool
This method of differentiation focuses on the attribute of the source port number. In this approach a single pool of potential port numbers is set aside for the purposes of NAT. As a pool number is assigned, it is removed from the pool so that two sessions from different computers can not using the same port number. Once the session is over and no longer in use by the computer, the port number is put back into the pool where it can be assigned again.
| Example global pool: | ||
| Hexidecimal | Decimal | |
|
Start or range |
0x7000 |
28672 |
|
End end of range |
0xF000 |
61440 |
|
Possible ports in range |
215 |
32768 |
This is a simple approach to implement and is good if the number of connections in unlike to reach the pool size. It would be okay for home use, but our example is for a university using 10.1.1.0/8 as a subnet. That means 16,777,214 possible IP addresses; more than this method can handle.
Fortinet does not use this method.
Global per protocol
This method uses the attributes source port number and type of protocol to differentiate between sessions.This approach is a variation of the first one. An additional piece of information is refered to in the packet that describes the protocol. For instance UDP or TCP. This could effectively double the number of potential addresses to NAT.