I'm trying to operate our Microsoft SMTP server and want to deny access to an entire class A block of ip addresses. I know there are probably other ways to do this but I want to use my Windows 2003 R3 default SMTP and POP3 server for mail processing. Getting lots of Russian hits we don't want. I'd prefer to do so this way instead of using Windows Firewall.
This is not a relay question.
But I'm a bit confused as exactly how to format the ip address class A block in the SMTP server I want to deny. I opened the SMTP server properties, ACCESS tab, then connection tab, using "All except the list below" and using Group of computers" to enter my ip and subnet.
Example. I want to deny all incoming ip addresses for the entire class A block beginning with 5. (such as 18.104.22.168) but I want every single IP address blocked in the entire class A. I'm aware of the ramifications and will do this in my SMTP server. How should it be written, including subnet?
Again, I am aware that it will block ALL ip addresses in the class A. I just need to be sure I'm entering the correct ip format.
As an example, if I want to deny smtp access to the entire class A block that begins wth 5:
IP Block 22.214.171.124. Subnet 255.255.255.255
OR THIS WAY?
IP Block 126.96.36.199. Subnet 255.0.0.0
Or some other variant? As you can see, I'm not sure if the "0" allows or denies. I'm an obvious beginner.