我们都知道在 TCP/IP 的架构下，所有想连网络的电脑，不论是用何种方式连连网络，都必须要有一个 *唯一* 的 IP-address。举两个最常见的例子: 学校的电脑用网络线连在一起，是经由 ethernet 接口连连网络，则该 ethernet 接口必须要有 IP-addr., 如 freebsd.phys.ntu.edu.tw的 ethernet IP 为 140.112.52.20 。另一个常见的网络接口为 PPP, 通常是用在 modem 拨接上，当您从家里透过 modem 拨接上远端的 modem server 时，有些 server 会从中建立起远端与您的电脑间的 PPP 连线协定，于是，您的电脑就会有一个 PPP 接口，而它的 IP-addr. 则由远端 server 指定给您 (这一切都是全自动进行) ，于是您就可以透过 modem (PPP) 对外连线，就好象您的电脑直接接在网络上一样。

-We knew that under the TCP/IP construction, all wants to link the network the computer, no matter with what way again and again network, must have* only* IP-address. Cites two most common examples: The school computer with the network line in the same place, is by way of continually the ethernet connection again and again network, then this ethernet connection must have IP-addr., like freebsd.phys.ntu.edu.tw s ethernet IP is 140.112.52.20. Another common network interface is PPP, usually is uses in modem dials joins, when you dial from the family by modem joins the far-end modem server, some server will establish the far-end with between yours PPP segment agreement computer s, therefore, your computer will have a PPP connection, but its IP-addr. Assigns by far-end server for you (all these is completely automatic carries on), therefore you may penetrate modem the (PPP) foreign segment, probably your computer directly meets in the network is the same.