以太网的最小帧长为（ ）字节，这个帧长是指（ ）的长度。
- A.40
- B.64
- C.128
- D.144
正确答案及解析
正确答案
解析
帧由8个字段组成，每个字段有一定含义和用途。 每个字段长度不等，下面分别加以简述。前导字段: 形为1010...1010，长度为7个字节。帧起始符字段：固定格式为10101011，长度为1个字节。目的地址、源地址字段：可以是6个字节。最高位为0，代表普通地址；最高位为1，代表组地址；全1的目标地址是广播地址。类型字段：标识上一层使用什么协议，以便把收到的MAC帧数据上交给上一层协议，也可以表示长度。类型字段是DIX以太网帧的说法，而IEEE802.3帧中的该字段被称为长度字段。由于该字段有两个字节，可以表示0 ~ 65535，因此该字段可以赋予多个含义，0~1500可以表示长度值，1536 ~ 65535 (0x0600 ~ 0FxFFF)被用于描述类型值。考试中 ，该字段常标识为长度字段。数据字段：上一层的协议数据，长度为0~ 1500字节。填充字段：确保最小帧长为 64个字节，长度为0 ~ 46字节。校验和字段: 32位的循环冗余码，检验算法见本书的CRC部分。注意:以太网的最 小帧长为64字节，这个帧长是指从目的地址到校验和的长度。
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