T636400000 does it have an ethernet card built in the unit?

Pretty much every computer built within the last 5 years has built on network interface. However, to answer your question completely - look at the back of the computer where the network cable plugs in? Is it clustered around the video/sound, power, printer, usb, etc (integrated in); or is it located elsewhere (perhaps in the series of long narrow rectangular openings openings where the expansion cards go - separate).

Hope this helps.

This is onr example of a Network adapter, or also know as an Ethernet Adapter Card,

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=686143&CatId=200

This is Dell Support >XPS 400 desktop computer > Service Manual,

http://support.dell.com/support/edocs/systems/xps400/sm/index.htm

1) Work on a table. Computer off, and unplugged from power, open the computer case.

(Removing the Computer Cover, in the list)

2) Follow Anti-Static precautions.

[Anti-Static Precautions:

Your body carries Static electricity.
Static will fry out (Short Circuit) the delicate hardware components, inside a computer.

To relieve your body of Static, TOUCH the metal frame of the open computer case.
Should you leave your computer in the middle of working on it, upon your return be sure to Touch the metal frame again ]

3) View the motherboard inside your computer.
(The motherboard is also referred to as the Mainboard, {Proper speak}, and System Board)

Technical Overview > System Board Components,

http://support.dell.com/support/edocs/systems/xps400/sm/techov.htm#wp1053398

Viewing the illustration of the motherboard, the Ethernet Adapter Card example I gave above, would go in either one of the PCI expansion slots.

These are white in color, and in the illustration are labeled - PCI Card Connectors.

If there is already an adapter card installed in both PCI expansion slots, then see if the PCI Express x1 expansion slot is open.

In reality, PCI Express is a better technology than PCI.

Shown on the motherboard as PCI Express x1 Card Connector.

This is an example of an Ethernet Card, (Ethernet Adapter Card), that uses the PCI Express x1 expansion slot,

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=4675156&CatId=200

The reason I have suggested buying, and installing an Ethernet Adapter Card is threefold.

1) You may have your Ethernet cable plugged into the Ethernet port of the computer,

http://support.dell.com/support/edocs/systems/xps400/sm/about.htm#wp1053567

[Number 3 - Network Connector],

and the built-in Ethernet is no good.
Reinstalling drivers May help, but the above is a sure thing.

2) You have an Ethernet Adapter Card plugged into one of the PCI slots, or the PCI Express x1 slot, and the Ethernet adapter card is no good.
They do go bad, and are economical to replace.

3) There was an Ethernet Adapter Card installed previously, but taken out before you received the computer.

It is buillt in however if you have a spare PCI slot on the motherboard you could fit another ethernet card which would override the built in one.

It is a driver issue... especially if your Device Manager has a question mark, and you recently re-installed windows.

This could be hardware failure as well, but looks a lot more like a simple driver problem. Find the latest driver, install it, restart, you should be good.

This unit is a desktop computer. According to the unit's specifications, the network adapters installed are RJ-45 - Ethernet Connector and RJ-11 Phone Connector. You can purchase a PCI Wireless Card to install in this unit if you do need wireless access.

Did you have it working with another computer before? It would normally use an ethernet cable between the router and laptop.
Your laptop needs an ethernet connector (or maybe a USB port if your router supports a USB connector). Sometimes the ethernet port is built into the laptop. It looks like a phone jack (you may have a couple of modem jacks on your laptop, which is not what you want). It's bigger and has 6 wires instead of 4. Another way to connect an ethernet cable to your laptop is a PCMCIA card. They plug into a thin, wide port on the side of your laptop. It also would have the 6-wire jack built into it. Router setup is another issue if you need help with that.

Also , not all IBM T20/T21 laptops shipped with a ethernet card fitted.
some had a modem only card. some a combined modem/ethernet card.

if the tiny LEDs light when plugged into a known active network connection then the card is present.
if they dont light up then it probably isnt present.

to check for the card..remove the panel secured by a single screw (just above center of the base)
the panel is in the top right quadrant of the bottom of the PC if you look at the bottom with the power connector at the top and the battery at the bottom.

remove the screw. open the panel.
there may or may not be a card present.
if there is if there is only a single white connector (with 2 small wires) then it is the Modem only card.
if there are 2 connectors one with 2 wires and another bigger with 8 wires..then you have the network card. check the connectors a fully pushed home.
its fiddly good luck

An Ethernet adapter is a network card adapter that allows you to connect another computer that also has an Ethernet adapter using a network cable to transfer files and allow remote access to the second computer. It is also used to plug in from a cable modem to allow you Internet access through you provider. You might already have one built in to your computer. Look for a small connector that resembles a telephone connector but it is a little bit bigger in size which is typically located on the back of the computer. Hope this helps.

without the port you cant connect

Joecho,
First question is, what version of OS X are you running? Second,
Can you go into your system preferences, select network prefs, and next to SHOW, choose Network Status and verify
That BUILT-IN ETHERNET is recognized?

Keep me posted.