Mail may come from machines which are identified using their host name or sometimes using their IP number. It is sometimes more practical to classify machines based on their IP number instead of their domain name. In the following section you will see that sometimes you are allowed to enter a name, sometimes an IP number, and sometimes both.
You can specify a network number by simply entering the prefix of the IP number for that network. For example, for the class C network 192.168.1.0, you simply enter 192.168.1. (note the trailing dot).
To specify a host name, you enter the fully qualified name, like machine1.domain.com. To specify a domain, simply enter it. This means that any machine that is part of this domain will be processed in the same way.