防火墙系统通常由(57)组成，防止不希望的、未经授权的通信进出被保护的内部网络。

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The　major　problem　with　E-mail　is　that　it　is(71)easy　to　use　that　people　can　become(72)with　messages(73)they　can　possibly　answer　in　a　day.　In　addition,　mail　boxes　require　some　management　to(74)messages　or　archive　those　that　might　be　required　later.　Senders　don't　always　know　about　your　E-mail　backlog　and　often　send(75)messages.

(73)

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(71)

DOM　is　a　platform.　and　language　-(66)API　that　allows　programs　and　scripts　to　dynamically　access　and　update　the　content,　structure　and　style.　of　WWW　documents(currently,　definitions　for　HTML　and　XML　documents　are　part　of　the　specification).　The　document　can　be　further　processed　and　the　results　of　that　processing　can　be　incorporated　back　into　the　presented(67)DOM　is　a(68)-based　API　to　documents,　which　requires　the　whole　document　to　be　represented　in(69)while　processing　it.　A　simpler　alternative　to　DOM　is　the　event　-　based　SAX,　which　can　be　used　to　process　very　large(70)documents　that　do　not　fit　into　the　memory　available　for　processing.