Thanks for VMRoyal and Tom's kindly reply.
In fact, we've a production environment which is running vSphere 5.1 without DNS service.
There're VM x20 living in ESXi x3 controlled by vCenter x1, and everything is working fine.(NFS/iSCSI and vMotion/HA/DRS/FT)
Would you please tell me which functions or features of vSphere 5.1 can't work without DNS service ? (exclude AD integration)
or which functions will work better with DNS service, not only for easy recognize than IP address by human beings.
Appreciate for your kindly reply.