Hey Jan,
Thank you!
You are right - your cameras will have to have fixed IP for this to work. IPs assigned automatically by your router (DHCP) usually change after a while - unless configurable otherwise. That is why all IP cameras allow you to set up a fixed IP address.
When your camera connects to your network it will request the IP address you assigned to it. All you have to do is to make sure that this IP address is not assigned to another device on your network already. Routers limit the range of IP addresses they use for the DHCP service - usually it is the first 100 or 200 addresses. E.g. if all your devices have IP adresses between 192.168.1.20 and 192.168.1.40 you are safe to assume that the IP 192.168.1.201 will never be used by your router unless assigned to a device manually.
You can use IP addresses all the way up to .253 for your fixed IP devices.
