摘自计算机网络原理书(前言): Thisbookisnowinitsfourthedition.Eacheditionhascorrespondedtoadifferentphaseinthewaycomputernetworkswereused.Whenthefirsteditionappearedin1980,networkswereanacademiccuriosity.Whenthesecondeditionappearedin1988,networkswereusedbyuniversitie