你可知RJ45不是我真名

RJ 是Registered jack的意思。是ANSI/TIA-968-A标准里面定义的。
我们常用的以太网的水晶头是8P8C的头，意思是8个槽位，8根线。但是，同样是8P8C,有
RJ38X: 8P8C, similar to RJ31X, with continuity circuit
RJ41S: 8P8C keyed, for one data line, universal
RJ45S: 8P2C + keyed, for one data line with programming resistor
RJ48S: 8P8C, for four-wire data line (DDS)
RJ48C: 8P8C, for four-wire data line (DSX-1)
RJ48X: 8P8C with shorting bar, for four-wire data line (DS1)
RJ49C: 8P8C, for ISDN BRI via NT1
RJ61X: 8P8C, for four telephone lines
这么多种，但是却唯独没有叫做RJ45的。怪异吧

唯一一个名字上和RJ45沾边的是
RJ45S: 8P2C + keyed, for one data line with programming resistor
却不是以太网的水晶头。

再看我们用的电话线的水晶头
模拟电话线的水晶头是6P2C， 你扒开一个看看，6个槽位，用了中间两根线。
RJ11C/RJ11W: 6P2C, for one telephone line (6P4C with power on second pair)
数字电话是6P4C，但是很多时候话机交换机都支持8P4C。
RJ14C/RJ14W: 6P4C, for two telephone lines (6P6C with power on third pair)

电话的听筒线，是4P4C，坊间的叫法都是不正式的，说白了就是瞎叫。淘宝上面叫RJ12，整个一乱弹琴。
"RJ9", "RJ10", "RJ22": 4P4C or 4P2C, for telephone handsets. Since telephone handsets do not connect directly to the public network,
they have no Registered Jack code whatsoever.