采用可变长子网掩码技术可以把大的网络分成小的子网，例如把子网掩码为255.　255.0.0的网络40.15.0.0分为两个子网，假设第一个子网为40.15.0.0/17，则第二个子网为(56)。假设用户X1有2000台主机，则至少应给他分配(57)个C类网络，如果分配给用户X1的网络号为196.25.0，则指定给X1的子网掩码为(58)；假设给用户　X2分配的C类网络号为196.25.16.0～196.25.31.0，则X2的子网掩码应为(59)：如果路由器收到一个目标地址为11000100.00011001.01000011.00100001的数据报，则该数据报应送给(60)用户。

(102)

(103)

(100)

(101)

Inside　the　computer,　the　operation　system(of　another　software　module　called　a　data　communication(71))must　decide　which　program　will　get　control　next.　Each　program　remember,　is　associated(72).　　a　particular　front　-end　port.　If　a　program's　data　have　not　yet　reached　its　front　-　end　buffer,　there　is　no　point　giving　that　program　control.　Thus　the　data　communication　monitor　sends　its　own(73)signal　to　the　front　-end　devices　next　port　is　polled.　Note　that　the(74)computer　speed.　The　computer　never　has　to　wait　for　a　terminal　or　a　communication　line.　Instead,　it　is　the　expressive　front　-　end　processor　that　waits　for　the　slower　system(75)to　respond.

(97)

(98)

(95)

(96)

Software　engineering　is　the　science　and　art　of　specifying,　designing,　implementing,　and(66)with　economy,　timeliness　and　elegance,　programs,(67)and　operating　procedures　whereby　computers　　can　be　made　useful　to　humanity.

This　definition　implicitly　sets(68)the　basic　goal　of　software　engineering　research　and　development:　　The　basic　goal　of　software　engineering　is　to　facilitate　specification,　design,　implementation　and　evolution　　of　programs,　documentation　and　operating(69)which　are　useful　to　humanity　in　an　economic,(70)and　elegant　manner(using　the　appropriate　science　and　art).