How to generate a random number in C#

I'm trying to generate a random number from 27 to 227 when a user signs up to be sent alerts on my website. I tried the random() function, but it seems to generate the same number each time. What am I doing wrong?

//var randomCode = new Random(27);

//validationCode = string.Format("{0}{1}","27", randomCode.Next(27, 227).ToString().Substring(0,3));