转自WIKIPEDIA
virtual IP address (VIP or VIPA) is
an IP address that is not connected to a specific computer or network interface card (NIC) on a computer. Incoming packets are sent to the VIP address, but they are redirected to physical network interfaces.
VIPs are mostly used for connection redundancy; a VIP address may still be available if a computer or NIC fails, because an alternative computer or NIC replies to connections.[1]