if they are plugged into the same switch or any bridge you can access them via any ip address unless it's conflicting. in that case just unplug and do it for one. write down all their mac addresses and turn them all to dhcp and then create static leases for them. the dvr software probably needs to know the ip of each somehow. subnets and what not don't matter at layer2. once you try and connect to the internet or an internal gateway you get problems. if you have any nested routers the NAT will block any thing trying to connect from the wan port. if the dvr has a NAT plug a computer into it's bridge to access the cameras.