学过网络的人都知道：同一级网络设备之间连接，须使用交叉线。 －－－－DP
但是现在好像没有人这么做了，大家两台电脑之间都是直连线完事儿，更有人认为直连线是天经地义的事情。其实不然，我们现在能够使用直连线连接同一级设备（如：两台pc），都是现在的网卡大多包含了一种叫做“自动网线线序识别“的功能，此功能是HP公司首创。此功能是由物理层芯片决定。只有网卡支持此功能，才能直连线连接2台PC，否则，还得使用交叉线连接，现在不支持此功能的网络设备也有不少。
来看看wiki上面是怎么说的吧：
Auto-MDIX
Auto-MDIX, which stands for Automatic medium-dependent interface crossover, is a computer networking technology developed by HP. It automatically detects the required cable connection type (straight-through or crossover) and configures the connection appropriately, thereby obviating the need for crossover cables to interconnect switches or connecting PCs peer-to-peer. When it is enabled, either type of cable can be used and the interface automatically corrects any incorrect cabling. For Auto-MDIX to operate correctly, the speed on the interface and duplex setting must be set to "auto".
Summary
Many local area network (LAN) products use a medium consisting of twisted pair copper wire pairs for the transmission and reception of data. It is generally a requirement that one or more pairs be used for transmission of data in one direction and one or more other pairs to receive data in the opposite direction. The connection between two node devices is a link. Many LAN devices, which employ a full-duplex medium, have different pairs for transmission than for reception and require, therefore, that each end of the link use a defined assignment for its twisted pair connector to a particular pair of wires. If the transmit pair of wires of one device is connected to the transmit connectors of another, obviously the communication link will fail.
Most LAN standards address this problem by assigning different connector pins to the wires in the twisted pair at the RJ-45, which is often referred to as the Media Dependent Interface (MDI). In switched networks, Network Interface Devices (NIC) may be directly connected to another NIC, or a repeater may be connected to another repeater, or a repeater may alternately be connected to a switch. Depending upon the assignment of pins in the network interface, it is sometimes necessary to use a crossover cable to address the problem that both products use the same pin designations on their interfaces. Determining whether the interface requires this cable can cause confusion and frustration if the customer lacks the necessary cable.
To get around this problem, some products use a manual "MDIX" switch. But that still requires user intervention. The need arose, then, for automatically ensuring viable connections among MDIs in complex LANs. HP's Auto-MDIX technology effectively solves the problem of lock step interaction among devices having MDIs from various well-known manufacturers
http://en.wikipedia.org/wiki/Auto-MDIX
http://www.hp.com/hpinfo/abouthp/iplicensing/automdix.html
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