需要在计算机网络的IP地址和物理地址之间建立映射关系，主要是由于(55)，在IP地址向物理地址映射时，使用的协议是(56)。

(82)

(81)

(80)

(79)

(77)

In　the　following　essay,　each　blank　has　four　choices.　Choose　the　most　suitable　one　from　the　four　choices　and　write　down　in　the　answer　sheet.　There　are　two　types　of　key　technology:　private　key　and　public　key.　Private-key　encryption　methods　are　called(71)ciphers,　information　is　encrypted　with(72)both　the　sender　and　receiver　hold　privately.　Public-key　encryption　methods　are　called(73)ciphers,(74)created　for　each　user,　encryption　key　and　decryption　key　are(75)

(76)

(75)

(74)

In　the　following　essay,　each　blank　has　four　choices.　Choose　the　most　suitable　one　from　the　four　choices　and　write　down　in　the　answer　sheet.　Heterogeneous　network　environments　consist　of　computer　systems　from(66)vendors　that　run(67)operating　systems　and　communication　protocols.　An　organization　that(68)its　computer　resources　is　usually(69)the　task　of　integrating　its　heterogeneous　systems.　Typically,　each　deparlment　or　division　has　defined　its　own　network　needs(70)OS,　LAN　topology,　communication　protocols,　applications,　and　other　components.