心血来潮，小水一篇！
回环地址（loopback address）
loopback 在维基百科上有一段这样的解释：Loopback (also written loop-back) is the routing of electronic signals or digital data streams back to their source without intentional processing or modification. It is primarily a means of testing the communications infrastructure.
通熟的说就是将由“源”发送出去的数据路由回“源”。
