The ACR9000 supports up to four simultaneous communication "Streams" via Ethernet communication. This allows communication with up to four different Ethernet devices provided that each device is only using a single stream.
Ethernet/IP, sometimes referred to as CIP (Control and Information Protocol) will use two streams, one for Class 1 (UDP) broadcast information, another for Class 3 (TCP) polled information, leaving only two streams available for connection to other devices.
More information can be found in the "Ethernet Specification: ACR Series Products" and the "Ethernet/IP Specification: ACR Series Products" found in our Literature section.
UPDATE: (dm 10/16/15)
With ACR firmware 1.25 and above and the IPA, only 1 connection is used with EIP from a PLC