iMac will not get IP address using DHCP on ethernet

When I connect an iMac to an unmanaged switch connected to a Cisco 2960, it does not acquire DCHP.  Previously connected to same network and worked fine, but now adding unmanaged switch in between creates this issue. Windows PC's connected to same switch are able to receive IP's from DHCP.  
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arnoldConnect With a Mentor Commented:
The network admin may have implemented the 801.x security model such that the system being connected fails the authentication and thus is not provided with access to the LAN (DHCP IP allocation)
Are the windows PC's members of the domain ?
What changed on the network?
jhyieslaConnect With a Mentor Commented:
Have you tried plugging into a different switch port or the same switch port as one of the PCs that works? Give it a static IP and then try and ping something to confirm connectivity.
danny8280Author Commented:
After searching through configs done by previous admin, found that port-security was set to maximum 2.  After two devices, nothing would connect.  Third device just happens to be Apple.
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