Two types of ethernet ports found on Macs:
(1) 10 Base-T looks like a big fat telephone jack
(2) AAUI-15 looks like a small SCSI port

Both types are marked with symbol <--->
Some Macs have both types of ethernet ports.

What you need to connect two Macs:
(1) For 10 Base-T port - Crossover Ethernet Cable (category 5 is best) - $12
(2) For AAUI port - an adapter called a 10 Base-T Transceiver - $20
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SETUP PROCEDURE:
1. Shut off both Macs.
2. Plug crossover cable into ethernet port of each.
3. Power up both Macs.
4. Open AppleTalk control panel on both Macs.
5. Select “Ethernet” in both. Close/Save.
6. Open File Sharing control panel in both Macs.
7. In Owner’s Name box, use same name for both Macs.
8. In Password box, use the same word for both Macs.
9. In the “Computer Name box”, assign a unique name for each computer (such as “Laptop”, “iMac”, etc.)
10. Click “Start” button in File Sharing for each computer.(Note: sometimes this takes a few secs,minutes)
11. Once file sharing is reported as On, close File Sharing control panel.
12. On the desktop of one Mac, click once on the Hard Drive icon.
--- Under File Menu, select Sharing and check the box.
---(This item may occur under Get Info in OS8).
--- Close and repeat above procedure on other Mac.
13. Open “Chooser” under Apple Menu on either computer.
14. On left side of Chooser, Click “AppleShare” icon.
15. Then on right side of Chooser, select computer you wish to connect to and click OK.
16. Dialog Box will appear asking for Password into other computer. Type it and click OK.
17. Click “Connect” in next dialog box.
18. Select which items to be accessible, such as HD.
---Close Chooser and repeat above for other computer.
---New icon Images should appear on each computer.
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---To disconnect, you can can turn off File Sharing in Control Strip at bottom.
--- A “People Warning” box will appear- type “0” and click OK. Repeat for 2nd Mac.
---Unplug crossover cable.
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(Note: The setup procedure only has to be done once.
Then is controlled by “File Sharing” on/off in Control Strip.

See How to Network 2 Macs Via Ethernet - OSX

See How to Network 2 Macs Wirelessly - OSX