Constraints

It is possible to make subdomain constrained.
For example if we want to make guid constraint on subdomain we could do:

public void Configure(IApplicationBuilder app)
{
    //...
    app.UseMvc(routes =>
    {
        var hostnames = new[] { "localhost:54575" };

        routes.MapSubdomainRoute(
            hostnames,
            "TenantInSubdomain",
            "{tenant:guid}",
            "{controller}/{action}",
            new { controller = "Home", action = "Index" });
    });
}

Now tenant can be only guid but you cannot use guid with curly braces since they are not allowed in hostname.
Constraints as an object are also supported: new { tenant = new GuidRouteConstraint()}.

Supported constraints:

Constraint	Comments
int	negative not allowed
bool
guid	curly braces not allowed
long	negative not allowed
minlength(value)
maxlength(value)
length(value)
length(min, max)
min(value)
max(value)
range(min,max)
alpha
regex(expression)
datetime	not allowed
decimal	not allowed
double	not allowed
float	not allowed

More info: Microsoft's Constraints Page