virtual IP address (虚拟 IP 地址)
1、是集群的ip地址，一个vip对应多个机器
2、与群集关联的唯一 IP 地址
see wiki：
A virtual IP address (VIP or VIPA) is an IP address assigned to multiple applications residing on a single server, multiple domain names, or multiple servers, rather than being assigned to a specific single server or network interface card (NIC). Incoming data packets are sent to the VIP address which are routed to actual network interfaces.
A server IP address depends on the MAC address of the attached NIC, and only one logical IP address may be assigned per card. However, VIP addressing enables hosting for several different applications and virtual appliances on a server with only one logical IP address.
VIPs have several variations and implementation scenarios, including Common Address Redundancy Protocol (CARP) and Proxy Address Resolution Protocol (Proxy ARP).
Usage
VIPs are mostly used to consolidate resources through the allocation of one network interface per hosted application.
It is also used for connection redundancy by providing alternative fail-over options on one machine; a VIP address may still be available if a computer or NIC fails, because an alternative computer or NIC replies to connections.[1]