Cross-over network cable

I have a cross-over network cable that I used to use to connect two computers using their network cards.

I now want to use that cable to connect a computer to a router. Is this not possible? Why?

Almost all recent network cards support auto-crossover (Auto-MDIX). With this feature, whether a cable is normal or crossover doesn't matter - you can connect two computers using a normal cable, so although I have not tested it I'm quite sure it would also allow connecting such a card to a router using a crossover cable.

If this feature is not supported by your network card, you'll have to use a 'normal' cable.

The other answers explain current solutions but I kinda felt like going into the original problem cause It allows me to reminisce on my time in the army as a cable dog.....sigh.....

The original problem simplified

Think about it like this. For your two pcs (or any alike devices really) your nic has a set of predefined "standard" dedicated directions for transmitting data and receiving data.(See 10baset and Category 5 for a more detailed look) So with that lets say two wire pairs are set to receive traffic and two are set to send.

|PC1 TX|------>X<-----|PC2 TX|
|PC1 RX|()<--- ? --->()|PC2 RX|

Because both sides are sending on the same line neither is getting anything. (Think of two water pipes trying to push water on both ends... )

Enter the router

However with a router the connections are set to the reverse.

|PC1 TX|------------>()|RTR RX|
|PC1 RX|()<----------- |RTR TX|

And with smarter routing devices your router can figure out what kind of cable you are using and act accordingly.

Yes this is a a bit of an oversimplification so forgive me if I left something out.

That would depend on a lot of things. Routers are generally made to use "normal" cables to connect to computers as they are wired to work that way. Most modern devices and network cards can use a normal cable as a cross-over cable or a normal cable. Unless all your devices are pre-1998 you should be able to use a cross-over cable with a router.

Wikipedia entry on Cross Over Cable and Auto-MDIX

Scenario 1

You have cat5e or Cat 6 cables. (Straight through (ST) or Cross Over (CO) Your switch is a gigabit switch that has the auto sensing Auto MDX standard. Your NIC card is a Gigabit NIC that has the MDX standard as well.

CO or ST cables do not matter in most cases since the MDX feature will auto correct the connection.

Scenario 2 Same as above but your NIC card does not have MDX standard. PC to switch with any cable. Switch to router any cable. Only the Switch can do the correcting. If the Router and the NIC card do not support MDX standard you would need a CO cable to directly connect the two.

Rule of thumb is CO for like devices BUT with the introduction of Auto-MDX in the Gigabit standard that CISCO rule of thumb is becoming an "old" standard that is being phased out with the newer technology.

The new tech is attempting to address all of the issues with ST and CO cabling and requirements in devices. With Auto-MDIX and other features in new equipment the needs of CO and ST technologies are becoming a thing of the past to help ease the issues in installation and upgrading. Down the road there will be just one cabling (Straight through)for cables.

The new rule of thumb is like this. If the equipment is newer than 2010 AND is Gigabit and the cabling is Cat5e or higher then you have a very good chance of having the ability to utilize Auto-MDIX. At least one device will to the auto crossover.

But it is still smart to keep at least one CO cable handy to test connections between older equipment or equipment you are not sure of. It is also good to know the older requirements and rules of thumb since we are still in a "transition phase" and you will find in many situations that there are a mixture of old equipment and new equipment and you may still need a few CO connections made for compatibility.