When you install VMware ESXi on a server, then first you need to add your license key through the web interface. Otherwise, you are using the trial version, which is valid for just 60 days. If you are downloading a free version of ESXi, then you can obtain license key by going to the evaluation page for ESXi.
VMware License Key
To view the license key, you will have to register with VMware, but if you have already downloaded the ESXi, then you must already have an account set up. After logging in, you have to click on the License & Download tab to view the key.
Once you get the key, then you have to load the web interface and then enter your login credentials there. There you have to enter your User name and password to do a successful login.
When you are logged in, then you will see the ESXi management interface. You should also view a message that you are using the software in assessment mode and it will expire in just 60 days.
To add a license for ESXi, you just have to click on the Manage under Host in the left-hand pane in Navigator. Now, simply click on the License tab and then you have to click on the Assign License link.
Now, you are in the popup dialog, there you have to copy and paste the license key into the text box and then simply click on the Check License.
After doing this, you will get a green checkmark and then tell you that the license key is valid for VMware vSphere X Hypervisor.
Now, simply click on the Assign License and you are done. If you ever want to upgrade your license to a paid product like ESXi Essentials, then you just have to come back on this page and add new license to delete the old one.