A production LUN on a VNX storage system has two LUN-IDs, an internal LUN-ID as well as a Host-LUN-ID. The Host-LUN-ID is the one you can define when you present the LUN to a host, i.e. add the LUN to a storage group.
One quick hint though. I often have to deal with a large number of similarly sized LUNs and to identify them without the need to verify IDs, I usually pick a slightly different size (e.g. 1 GB). I'm doing the same for virtual disks, if a VM has multiple disks.
André