需要在计算机网络的p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)。

(74)was designed many years ago，with a goal in mind：transmitting the human voice in a more-or-less recognizable form．

In 8011 terminology．all communication was to go through the base station, called an(75)

(82)

In the data link layer, the data unit is called(71)．

The(72)is a system for linking hypertext documents Originally, each document was a page written in(73)with hyperlinks to other documents．Nowadays，XML is gradually starting to take over from(73)．

(79)

(78)

(76)

TCP/IP是一个协议簇，它的体系结构分为4层：应用层、网际层、网络接口层和(47)，其中，ARP属于(48)。为了在源主机和目的主机之间传送数据，IP需要确定源主机和目的主机是否在同一个网络中。如果不在同一网络时，则必须通过(49)进行通信。

(75)