when we're connecting ethernet devices we're commonly using a cable that is a straight through cable this means that the wire on pin one on one side is connecting to the wire on pin one on the other side pin two to pin two pin three to pin three and so on you'll sometimes hear this referred to as a patch cable because this is the cable we use inside of a wiring closet to patch from a patch panel into an ethernet switch this is a very common ethernet cable if you were to look at the ethernet cable being used on your computer it's probably a straight through cable and it's most commonly used to connect workstations to network devices such as switches this is a wire map of tin base t and 100 base t straight through cables you can see that there are only two pair being used here because only two pairs of wires are used for 10 megabit and 100 megabit networking this block on the left is a network interface card this is the adapter that's inside of your computer and it's technically referred to as a media dependent interface or an mdi you can see that pin 1 is a transmit plus pin 2 is a transmit minus pin 3 is a receipt plus and pin 6 is a receive minus you can see that with 10 and 100 megabit ethernet we don't use pins 4 5 7 and 8. on the right side is a network switch this is a media dependent interface crossover or mdi-x sometimes we'll refer to this as mdx this is a receive plus and receive minus on pins one and two and transmit plus and transmit minus on pins three and six this means that pin one transmit on one side is connected to pin one receive on the other side if we were to look at the traffic flows across this 10 or 100 megabit ethernet cable the network interface card would be transmitting to the receive pins and receiving traffic from the transmit pins on the other side this traffic flow changes a bit when we get into gigabit networking and higher with a gigabit straight through or 1000 base t straight through connection between a network interface card or mdi communicating to a network switch or mdi-x you can see that there is no longer a transmit and receive side that's because with gig communication there's both transmit and receive on every single wire used and we're using all four pairs of wires in a single ethernet cable in this diagram i've labeled the pairs of wires as data a data b data c and data d this is why whenever we're connecting a gigabit ethernet cable we have to make sure that all eight wires are connecting on both sides of the cable if we're connecting two devices to each other that are both mdi devices for example two separate workstations to each other or we're connecting mdix devices to each other for example connecting two switches to each other we would not use a straight through cable instead we would use a crossover cable this is the crossover cable pin out for 1000 base t based on the 802.3 standard you can see that pin one goes to pin three pin two to six three to one four to seven five to eight seven to four and eight to five because all eight of the wires are used in a gigabit communication we would also need to cross over all eight of those wires to create a crossover cable one nice feature on most modern ethernet devices is that it can automatically identify when a crossover cable is needed and implement what's known as auto mdix or auto index this means that both devices will identify that they're both mdi devices or both mdix devices and internally automatically crossover the communication without needing a separate physical crossover cable this certainly simplifies things if you're connecting to a network because you can plug in a single straight through cable and the devices themselves will determine if a crossover is needed or not one common misnomer with crossover cables is that one side of the crossover cable is wired to be tia 568a colors and the other side is wired to be tia 568b colors well as you can see with this crossover cable for 1000 base t that's actually not the case one side is configured in this case for 568b but the other side is neither 568a or b colors the determination of what colors are used on each pin come from the tia-568 standard but the determination of what makes up a crossover cable comes directly from the ieee 802.3 ethernet standard this confusion with a crossover cable being 568a on one side and 568b on the other probably started when we only had two pairs of wires that we needed to worry about and with 10 and 100 megabit ethernet you could see that the wires on one side and the wires on the other side do have a similarity to a and b wiring obviously we're not using pins four five seven and eight this definition falls apart though when we get into gigabit communication because obviously a 1000 base t crossover is not 568a on one side and 568b on the other i realize there are many books and many websites that will tell you that a crossover cable is 568a on one side and 568b on the other but as you can see that isn't actually the case when you get into gigabit networking so if you're connecting devices together do you use a straight through cable or do you use a crossover cable if you're connecting a workstation to a switch this would be an mdi device connecting to mdix we would use a straight through cable if you're connecting a router to a switch it's a similar configuration with a straight through cable if you're connecting a switch to a switch those are both like devices which means we would not use a straight through cable we would use a crossover cable a router to router is also a scenario where both devices are identical we would use a crossover cable with those as well and the same applies when connecting a workstation to a workstation because those are the same device we would use a crossover cable a workstation to a router looks like these would be two separate devices but both a workstation and a router are both mdi devices so you would use a crossover cable with that configuration as well you