The first part of the question is more difficult to answer. Each site is free to choose subnet masks for its networks. When making assignments, managers attempt to balance sizes of networks, numbers of physical networks, expected growth, and ease of maintenance. Difficulty arises because nonuniform masks give the most flexibility, but make possible assignments that lead to ambiguous routes. Or worse, they allow valid assignments that become invalid if more hosts are added to the networks. There are no easy rules, so most sites make conservative choices. Typically, a site selects contiguous bits from the local portion of an address to identify a network, and uses the same parti- tion (i.e., the same mask) for all local physical networks at the site. For example, many sites simply use a single subnet octet when subnetting a class B address.