The link below takes me to my license key for ESXi 4.1. The key says it's good for 4.1 or later. Is this the same key I use for ESXi 5.1? I can't find a key for 5.1 on the 5.1 download page. There's no "manual download" button to select and no key in sight.
Yes Since it says ESXi 4.1 and later it should work for 5.1 too.
To Assign the License key for standalone ESXi follow the steps below.
To license a standalone ESXi 5.x (vSphere Hypervisor):
for every major version of ESXi you need a specific key. That means that a 4.x key will only work for 4.0 and 4.1. If it is a paid version and you got SnS active you can upgrade the key(s) in your MyVMware account.
If you are using the free Hypervisor you have to create a new key for your ESXi 5.x host.
There is a field called "License Version" when you decrypt license keys. This shows where the license keys are applicable. For all 4.x based licenses, the LicenseVersion = "4.0". For all 5.x based ESX, LicenseVersion = "5.0". So you cannot use them interchangeably. You need to purchase a license for your 5.0 host or go with the free hypervisor license (in this case, you won't be able to add it to vCenter).
Hope this helps..