HOW TO: Create a Custom Route Constraint in ASP.NET MVC
Solution
Creating a Custom Route Constraint (C#)
by ASP.NET Team
Custom Route Constraint in ASP.NET MVC
by Keyvan Nayyeri
ASP.NET MVC Tip #30 - Create Custom Route Constraints
by Stephen Walther - examples of creating an authenticated constraint, a not equal constraint, and a local machine constraint.
ASP.Net MVC Route Constraints
by Guy Burstein
Related Resources
MSDN: IRouteConstraint Interface