Ping to broadcast address fails (OS X 10.11)

With an ethernet camera or a Powerbook connected to Ethernet 1 of my mid-2012 Mac Pro, I can't see the connected device by using

ping 255.255.255.255

I get "ping: sendto: No route to host".

This command needs to work so that I can tell what's connected to the computer without knowing the device's IP address.

Edit: I should add that I turn off WiFi before running this Terminal command.

If I do the same thing with the same hardware setup under Yosemite (external bootable drive) or Mountain Lion (internal partition), the connected device responds.

When I use Wireshark I see both outgoing and incoming messages under Yosemite & ML, but neither message under El Capitan.

In either case, 10.10 or 10.11, using appropriate network settings for IP address, the ping works when I send it to the device's IP address rather than 255 etc.. So the port works but it's the broadcast ping that's broken.

I've never seen this exact problem posted, but have tried various remedies for "ethernet not working", mostly related to throwing out pref and system config files.

Has anyone else seen this?