When a device has a static IP address and it is introduced to a new network with a different IP scheme it is possible to gain access to the device inorder to reprogram it. First keep in mind that the subnetmask determins the network a device is connected NOT the physical connections to a switch. This means you can isolate systems on the same physical network by giving them different “Network” addresses, meaning the masked portion of the IP. The network portion or masked portion of the IP must match inorder for two devices to communicate. No, you can not simply open up the netmask on just one of the devices.
In this example assuming a subnet mask of 255.255.255.0, we have 3 networks: To gain access to all 3 networks you could change the subnet on the devices you want to communicate to 255.255.0.0. This will not work in all situations, for example if you introduce a device with an IP of 192.168.5.199. Also changing the subnet may not be an option. The solution is to simply add multiple IP addressses to the PC you have access to. When working remotely you will first need to assign a static IP to the device you are connected to. Be sure to including DNS and GW. Next, you will add a IP on the same network as the device you are connecting to but you will only add the IP and subnetmask. Do not add a gateway as a second GW could route your remote connection to a dead end and cause you to become disconnected.