A question about Ethernet cables…

[cotfessi]

I was hoping someone might definitively explain to me what the difference is between a crossover and a regular Ethernet cable. Here is the reason that I ask:

I’ve had SDSL for about two years now and for that time I’ve had a cable going from my DSL modem to a 4 port Netgear hub. I push in the uplink button on the hub and then I plug three other computers into the remaining ports and I am able to connect to the internet from the rest of the machines.(I have three static IP addresses) Now as I said it’s been two years since I set this up, but I vaguely remember that I needed a crossover cable between my hub and my DSL modem and regular Ethernet cable between the hub and the NICs of the other machines.

Now someone gave me an old machine and I’ve decided to put smoothwall on it and run a firewall/proxy/etc. So instead of DSL modem to hub, I’m going to run DSL modem to the first NIC in my new smoothwall box. So I decided to use the same cable that used to go from DSL modem to hub… but it didn’t work.

Now I’ve gone through every possible thing from the software and hardware on my firewall box – both NICs are brand new and work in other machines, all configurations ( ip, gateways, netmask, dns, etc.) are correct. So I called my DSL tech support and he said to check the cables…

Now I’m not at home right now, so I can’t try it, but what is the difference between the two cables?

Thanks in advance,
- cotfessi

p.s. If anyone hasn’t tried it yet, I would highly recommend Smoothwall (http://www.smoothwall.org) – They have a pretty slick product – version 2 runs the 2.4 kernel, a web proxy, DHCP server for your internal network, squid, and snort. All of the administration is handled via a very well laid out web interface. It is a very stripped down version of Linux, but if you’re going to use a box for just a firewall, that’s really all you need!

[Satanic Atheist]

CAT5 cable consists of 4 pairs of wires, with each pair twisted around eachother. The way that they are connected are either "straight through" cables in which each pin on each plug is connected to the same pin on the plug on the other end.

Crossover cables have the same setup, but two of the pairs are swapped on one end.

The end result is that you can connect two machines without a hub, switch or router with just a crossover cable. Otherwise, you will need one of these peripherals.

If you have only limited use for such a cable, I suggest that you get a crossover adapter which plugs into the cable and the NIC and will cross the required pairs over to allow you to either network normally or point-to-point at your choice.

James

[cotfessi]

nevermind... I found something on the net at http://discountcablesusa.com/ethernet-cables.html


Crossover Cables vs Straight Through Cables

Ethernet patch cables can be wired in three different ways, the two main ways are called straight through and crossover. The third type is called rolled and has only specialized applications.

Generally speaking, straight through cables are used to patch between different types of equipment; for example, PCs to a hub.
Conversely, crossover cables are generally used to patch between similar types of equipment; a PC to another PC for example.
Some modern hubs don't care if you use crossover cables or straight through cables, they work out what you're using and configure themselves accordingly.

As stated at the outset, the actual difference is in the wiring. Inside the UTP patch cable there are 8 physical wires although the network only uses 4 of them (the other 4 are simply wasted). The 8 wires are arranged in what's known as pairs and one pair is used to send information whilst the other pair is used to receive information.

On a PC, the pair on pins 1 and 2 of the connector send information, whilst the pair on pins 3 and 6 receive the information. To make PCs talk to each we therefore need to connect the send pair of one PC to the receive pair of the other PC (and vice-a-versa). That means we need a crossover cable. If we used a straight through cable the both be listening on the one pair - and hearing nothing, and sending on the one pair - achieving nothing.

Electrically, the straight through and crossover cables look like the diagram below: The most common cable is the straight through cable. In a home or small office network you might only have one crossover cable used - perhaps from the cable or DSL modem to the distribution hub...