A LAN is used for communications in a small community in which resources,
such as printers ,software and servers, are shared.Each device connected to a LAN has a unique address.
Two or more LANs of the same type can also be connected to froward data frames among multiple
users of other local area networks. In LANs, frames are additional headers appended for local routing.
These new-looking packets are known as packets.
（1）The Internet is a global system of interconnected computer networks that use thestandard Internet Protocol Suite (TCP/IP) to serve billions of users worldwide.
翻译：互联网是相互连接的计算机网络的全球系统，该网络使用标准的互联网协议套件（TCP / IP）为全球数十亿用户提供服务。
（2）As with most application-layer protocols, SMTP has two sides: a client side, whicheutes on the sender's mail server, and a server side, which executes on the recipien's mailserver.
翻译：与大多数应用层协议一样，SMTP有两个方面：客户端（在发件人的邮件服务器上使用）和服务器端（在收件人的邮件服务器上执行）。
（3）In the star topology, all users are directly connected to a central user through twounidirectional links: one for uplink and the other for downlink.
翻译：在星型拓扑中，所有用户都通过两个单向链接直接连接到一个中心用户:一个用于上行，另一个用于下行。
（4）A computer network is a group of computers that are connected to each other for thepurpose of communication.
翻译：计算机网络是出于通信目的而相互连接的一组计算机。
（5）WANS are used to connect LANs and other types of networks together, so that users andcomputers in one location can communicate with users and computers in other locations.
翻译：WANS用于将LAN和其他类型的网络连接在一起，以便一个位置的用户和计算机可以与其他位置的用户和计算机进行通信。