Jack Vamvas explains how to register an SPN for SQL Server:
I received a question from a colleague asking how to register a Service Principal Name (SPN) for SQL Server. The specific situation relates to upgrades and new SQL Server instances deployed onto new servers\server names .
Defining an SPN results in an Active Directory name – allowing a client to uniquely identify the service instance. The SPN is comprised of a service name with a computer and user account resulting in a Service id.
Read on for the process.