vCenter Server 5.x requires a license server to manage licenses on ESX/ESXi 3.5. For ESX/ESXi 4.x and later hosts, vCenter Server 5.x does not require a license server.

If you do not have a license server installed and you need one, download the VMware License Server from the VMware Web site.

The license server installation requires no downtime. No virtual machines, servers, hosts, or clients need to be powered off for the installation of the license server.