发送电子邮件时，通常使用的协议是(63)，通常使用的端口号是(64)。

(74) is a connectionless TCP/IP transport layer protocol.

(75) is a TCP/IP application service that converts user-friendly names to IP address.

A(72) is a data communication system that allows a number of independent devices to communications directly with each other in a limited geographic area.

(73) is a multimedia Internet service that allows users to traverse the Internet by moving from one document to another via links that connect them together.

In(71), multiple transmission can occupy a single link sectioned by time rather than frequency.

(67)

(68)

(65)

局域网中应用最广泛的差错控制方法是(47)校验。在CRC校验中，假设采用的生成多项式为4阶多项式，它产生的校验码为(48)位。在接收端，若发现错误，则将采取(49)措施。

(64)