Citrix XenServer is available in multiple editions, including a free edition. The Free XenServer edition has no limits on the number of VMs which can be run on it, and includes production grade features like shared storage, resource pools, centralized management and live migration of VMs between hosts in a pool. When you first install Free XenServer, a 30 day trial license is included. If you wish to continue to use XenServer after this 30 day period, you will need to either obtain a premium license, or activate your Free XenServer.
Activation is a simple process which starts from the XenCenter License Manager. Depending on your version of XenCenter, the License Manager could be either under the Tools menu or the Server menu. All activation requests start from XenCenter. If you are not normally using XenCenter, you will need to do so at least during the activation process.
The activation process starts with the selection of XenServer hosts to activate. Multiple hosts can be activated at the same time, and for each activation request you will receive a single license file valid for the XenServer hosts contained in the activation request. This license file will be sent to you via email after filling out a registration form. Typically this email is sent within 30 minutes of your request, though during non-business hours this may be longer due to system maintenance.
The vast majority of activation requests initiated by XenCenter will contact the XenServer activation server without requiring manual intervention. The most common situation when this wouldn't occur is during either an outage of the activation system, or due to network issues or restrictions between your instance of XenCenter and this activation server. In order to accommodate for this situation, XenCenter will prompt you to save the activation data, and then request you upload it to https://activate.vmd.citrix.com. This page is that site.
To obtain your XenServer license file, please upload the activation file saved from within XenCenter. This is the file you were prompted to save when automated activation failed.
Note: If you are attempting to renew your Free XenServer license using this site, the required file must be a recent activation file generated by XenCenter. Expired license files are not valid for this process. As with initial activation, the process begins with the XenServer License Manager.