學過網絡的人都知道：同一級網絡設備之間連接，須使用交叉線。 ? －－－－DP

但是現在好像沒有人這么做了，大家兩臺電腦之間都是直連線完事兒，更有人認為直連線是天經地義的事情。其實不然，我們現在能夠使用直連線連接同一級設備（如：兩臺pc），都是現在的網卡大多包含了一種叫做“自動網線線序識別“的功能，此功能是HP公司首創。此功能是由物理層芯片決定。只有網卡支持此功能，才能直連線連接2臺PC，否則，還得使用交叉線連接，現在不支持此功能的網絡設備也有不少。

來看看wiki上面是怎么說的吧：

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

別想當然：關于交叉線和直連線的使用，