Что такое обратная маска:

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WildCard-маска - это обратная маска. Берем шаблонное выражение: 255.255.255.255 и отнимаем от шаблона обычную маску.
255.255.255.255-255.255.255.0, у нас получается маска 0.0.0.255, что является аналогом обычной маски 255.255.255.0, только 0.0.0.255 является WildCard маской.
Т.е. если в обычной маске у нас написано 255.0 это значит что числа слева от точки не могут изменяться, а справа могут. Для обратная маска для такого случая будет 0.255.
Более подробно:
http://habrahabr.ru/post/131712/

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