Network Connection ID shall not be reused until the connection has been closed or has timed out

When the EtherNet/IP device is restarted it might use a network connection ID that was used for a connection that was runnung just before the restart.

This is what the specification says (CIP Volume 2 Edition 1.17, chapter 3-3.7.1 "Network Connection ID" ):
The Network Connection ID shall not be reused until the connection has been closed or has timed out. When a device restarts, it shall not reuse Network Connection IDs from previously opened connections until those connections have been closed or have timed out. A specific connection ID shall not be reused so long as there is the possibility that packets with that connection ID are present in the network.