Change of Network Service Stopped OSI Layer-2 Communication

Prior to the incident, all PLCs of a control system were connected to one network service (i.e. one port of a router). Over this service, all were also able to use the OSI layer-2 (ISO) protocol for communication. Due to an exhaustion of all IP adresses in the subnet, the PLCs were regrouped into two services by connecting some of them to another port of the same router. Unfortunately, due to the fact that the ISO protocol was not bridged between the two services, the inter-communication between the PLCs of both groups ceased.

The production control system was off for some days.