An IP Cam must be connected to a network, via WiFi or via Ethernet to the router. If the connection is done via direct Ethernet, there won't be any router, so the IPs need to be manually configured to work in the same network.

In order to achieve this, we can fix the IP of the camera, and set an static IP on the client.

The IP of the camera can be set in the camera settings (e.g. web interface).

The static IP for the raspberry can be set like so:

# uncomment if the example is set
interface eth0
static ip_address=192.168.1.10/24 # (for IP 192.168.1.10)
static routers=192.168.1.1
static_domain_name_servers=whateva