Hello Umesh,
Port block allocation is also a type of port address translation (PAT). It gives you more control on how many ports to be allocated to each lan IP from which the traffic is received on the firewall and
firewall should be NATing.
As explained in the article:
https://docs.fortinet.com/document/fortigate/7.2.6/administration-guide/29961/dynamic-snat
Ideally the number of ports available on an external IP is 60416, now in your case let's say you have defined Block Size as 128, means each block will be having 128 ports, and you have defined, Blocks Per User(which means blocks per internal user IP) as 8.
Above calculates to, each block has 128 ports and each user lan IP will be allocated 8 blocks, in total each user lan IP will be allocated 1024 ports with external IP while NATing, not anything beyond.
Now the question will be how many users can use one external IP with above configuration, so it will be 59 IPs. This is calculated as below:
Total available ports 60416, and each lan IP we are allocating 1024, so 60416/1024 = 59(user IPs).
With normal overload, preserved source port disabled, it's random.
It will depend on the FGT algorithm, if the session will use the same source port or change it.
We will never know.
Unless you use PBA wherein you define the port range that will be used by the source IP address.
Hope this helps.
