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http://www.linfo.org/rj-45.htmlRJ-45, short for registered jack-45, is an eight-wire connector that is used with Category 5 cable for connecting computers and other devices on local-area networks (LANs), especially Ethernets.
A LAN is a network that connects computers and other devices in a relatively small area, typically a part of a building or an entire building, usually for a business or other organization. Ethernet is the most widely used LAN architecture.
Category 5 cable, commonly known as Cat 5, is designed for use in high speed LANs. It consists of a thin plastic sheathing that contains four twisted pairs of copper wire, typically 24 gauge wire with three twists per inch. The twisting reduces electrical interference and crosstalk.
RJ-45 connectors look similar to the ubiquitous RJ-11 connectors, which are used primarily for connecting telephones, fax machines and modems, but they are somewhat wider and have eight contacts instead of four.
LANs originally used coaxial cable and BNC connectors. Coaxial cable typically consists of a single copper wire that is surrounded by a layer of insulation and then by a grounded shield of braided wire or an extruded metal tube. This, in turn, is usually wrapped in another layer of insulation and, finally, in an outer protective layer. BNC (Bayonet Neill Concelman) connectors were designed specifically for connecting coaxial cable. Cat 5 cable and RJ-45 connectors offer some important advantages over the coaxial alternative, including more flexibility, greater ease of use and lower cost.
RJ is an acronym for Registered Jack, because this interface is registered with the U.S. Federal Communications Commission (FCC). The RJ series of connectors (which ranges from RJ-10 through RJ-61) is primarily for use with telephone equipment, and the various models have a similar appearance but differ mainly with regard to the number of conductors and width.
RJ-45接口可用于连接RJ-45接头，适用于由双绞线构建的网络，这种端口是最常见的，一般来说以太网集线器都会提供这种端口。我们平常所讲的多少口集线器，就是指的具有多少个RJ-45端口。
集线器的RJ-45端口即可直接连接计算机、网络打印机等终端设备，也可以与其他交换机、集线器等集线设备和路由器进行连接。需要注意的是，当连接至不同设备时，所使用的双绞线电缆的跳线方法有所不同。
常用B标RJ-45线路连接顺序为橙白、橙、绿白、蓝、蓝白、绿、棕白、棕。当连接不同设备时，使用A标，其连接顺序为绿白、绿、橙白、蓝、蓝白、橙、棕白、棕。一般都常用B标的压法，如果要做交叉线，就一边为A标另一边为B标。
以下是各类设备的连接情况下，正线和反线的正确选择。(HUB代表集线器，SWITCH代表交换机，ROUTER代表路由器)。
PC-PC 反线
PC-HUB 正线
HUB-HUB普通口 反线
HUB-HUB级连口-级连口 反线
HUB-HUB普通口-级连口 正线
HUB-SWITCH 反线
HUB(级联口）-SWITCH 正线
SWITCH-SWITCH 反线
SWITCH-ROUTER 正线
ROUTER-ROUTER 反线
由于两者的尺寸不同（RJ11为4或6针，RJ45为8针连接器件），显然RJ45插头不能插入RJ11插孔。反过来却在物理上是可行的（RJ11插头比 RJ45插孔小）， 由此让人误以为两者应该或者能够协同工作。实际上不是这样。强烈建议不要将RJ11插头用于RJ45插孔。
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