isr.co.ro : glossary Address Mask A pattern of bits used to allow software to separate an Internet-style IP (Internet Protocol) address into the portion that represents the network and the portion that represents the host computer, local resources, or subnet--which is why it is also known as the subnet mask. This is one of the parameters you must enter when setting up most programs for accessing the Internet. You should get this value in the initial setup package from your service provider or support person.