What are two valid network prefixes in address books?

http://www.juniper.net/techpubs/en_US/junos12.1/topics/concept/zone-address-book-understanding.html

You can specify addresses as network prefixes in the prefix/length format. For example, 1.2.3.0/24 is an acceptable address book address because it translates to a network prefix. However, 1.2.3.4/24 is not acceptable for an address book because it exceeds the subnet length of 24 bits. Everything beyond the subnet length must be entered as 0 (zero). In special scenarios, you can enter a hostname because it can use the full 32-bit address length.