Why Gigabit Ethernet Direct Connect is not default at Macs? Trying to connect two Macintoshs with a Ethernet cable (capable of Gigabit speed, of course) will not automatically set a gigabit connection. Instead of this, a 100MB connection is established. Does anybody knows why? After a thousand searchs at google, and trying with a bunch of machines, I still on limbo.  Both ports are capable and set to Gigabit speed, imho. Thanks for any advice, answering or any way I should/could improve this question.

Both Macs tested are:
Both iMac (late 2011+), iMac + MacPro (Late 2009+)
All machines running Yosemite full updated.
Gigabit Ethernet is new technology in wired networking so if you're using a crossover cable, it will only work up to 100Mbps. Use a straight cable instead.

In Wikipedia:


  An Ethernet crossover cable is a type of Ethernet cable used to
  connect computing devices together directly. It is most often used to
  connect two devices of the same type: e.g. two computers (via network
  interface controller) or two switches to each other. By contrast,
  patch cables or straight through cables are used to connect devices of
  different types, such as a computer to a network switch or hub.
  
  Many devices today support Auto MDI-X capability, wherein a patch
  cable can be used in place of a crossover cable, or vice versa, and
  the receive and transmit signals are reconfigured automatically to
  yield the expected result.


To make things easier to understand, network interface cards (NICs) back then were "dumb" so they still need separate lines for receive and transmit signals. Now, NICs can identify both in one line.