导图
网络应用模型
客户/服务器(c/s)模型
P2P模型
DNS
域名
域名服务器 
域名解析过程 
文件传输协议FTP
FTP服务器和用户端
FTP工作原理
电子邮件
电子邮件的信息格式
组成结构
邮件服务器的功能:
1.发送&接收邮件
2.给发件人报告邮件传送结果
简单邮件传送协议SMTP
SMTP的缺点:
MIME
邮局协议POP3
网际报文存取协议IMAP 
基于万维网的电子邮件
万维网
超文本传输协议HTTP 
HTTP协议的特点
HTTP协议的连接方式:
HTTP-报文结构