Can I scan an ad hoc network to find all the connected devices and their IP addresses? I am trying to connect an IP Camera (with an onboard web server) to a Macbook Pro (OS 10.11). If I use a router it's very easy: they both connect to the router and then the Mac can log into the camera and see its image. 

I'd like to run on batteries and so I want to do away with the router.

I've set up an ad hoc network with Create Network, and my IP Camera may be logging in, but I have no idea at what IP address.

I've tried adr - a and ping 192.168.255.255 (because subnet mask is 255.255.0.0) but neither seem to work on an ad hoc network.

Is their another builtin or add-on tool I could use?

Thanks.
Normally devices should auto-assign themselves an address in the 169.254/16 subnet (i.e. 169.254.x.y) when connecting to an adhoc-network that doesn't offer a DHCP server.

You can use a tool like nmap from nmap.org to scan the subnet to find your camera.