We propose ExpressEther as Ethernet-based virtualization technology for a reconfigurable hardware platform. It groups modularized hardware resources interconnected by an Ethernet, and transports a PCI Express (PCIe) packet between the grouped modules by encapsulating it into an Ethernet frame. The configuration of the group is dynamically reconfigured by an Ethernet connection, followed by standardized PCIe hot-plug event. Such reconfigurability enables sharing of a physical resource among computer entities. We demonstrate that an I/O device is shared by servers, using our developed prototype consisted of an interface card and I/O concentrator. A commercially available server and serial ATA card are used for the demonstration, without any change for an operating system, device driver, PCIe interface, and Ethernet switch. The benchmark of I/O performance shows at most 16% degradation, which is caused by the implementation matters of our prototype. No degradation is measured when data flow from an I/O to a server.