use imac as secondary monitor

I want to hook up an old iMAC as a secondary monitor.  Is this possible?  If so, how do you do it.    See the screenshot attachment for the ports on the old iMAC and a screenshot of the specs of my new iMAC.
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Justin EvansCommented:
Make sure that your iMac is turned on, and the other Mac is logged in to a macOS user account.
Connect the Mini DisplayPort or Thunderbolt cable between the two computers.
Press Command-F2 on the keyboard of the iMac that you want to use as a display.

Or look at this youtube link

hope this helps

mkramer777Author Commented:
so what is the exact cable I need to do this?
The newer iMacs can use a display port cable, but, based on the 1st image there is no display port on this iMac.  Your 2 images seem to be in conflict.  The 1st image shows firewire 800 and firewire 400 ports that are available on a much older model of Mac.  I also don't see the standard Mini Display Port connector, just the old mini-DVI video port.  Your 2nd image shows a 2015 iMac which would no longer have those obsolete firewire ports or the older display connector.  Are the images from 2 different Macs?  The DVI port won't work for using your iMac as a monitor.
Eoin OSullivanConsultantCommented:
Bad news - the 2nd link posted by serialband shows that the oldest mac which supports the "Target Display Mode" is the following 2011 model which had an aluminium body and a thunderbolt port.   You appear to have an older model  from around 2008/09 so it is not an option

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Displays / Monitors

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