Actually that's a lot of questions in one
Let's keep things simple first.
When you just want to play together with 2 PCs, you don't need no dedicated server (yet).
So let's try to get this running first: 2 PCs playing IL-2 together, one hosts the game, the other one joins it.
You would want to check your network settings first.
20.0.0.0/8 networks are quite unusual since that network address space is reserved for CSC since 1994.
Depending on how many addresses you need, you would rather want a /16 (65,536 addresses), /12 (1,048,576 addresses) or a different /8 (16,777,216 addresses) block.
The IP address ranges are accordingly:
/16: 192.168.0.0 - 192.168.255.255
/12: 172.16.0.0 - 172.31.255.255
/8: 10.0.0.0 - 10.255.255.255
/16 will usually do, you would want PC1 be e.g. 192.168.0.1 and PC2 be 192.168.0.2
Now setup the PCs IP addresses accordingly and check they can "see" each other.
In order to do so, open a command prompt (Start -> Run -> cmd) and type "ping 192.168.0.2" on PC1 or "ping 192.168.0.1" on PC2 accordingly.
As long as that doesn't work it's pointless to proceed.
Check your Firewall for required settings (I don't know enough about ESET to tell you).
In doubt, disable the firewall completely at least for testing purpose.
Once you got this working, please reply so we can proceed.
Best regards - Mike