How Do I Make Sure My Random Number Does Not Change When I Refresh The Page?

See more:

C#

ASP.NET

MVC5

This is the code i am using to generate a unique student number. It successfully generates the 4 digit random number, however when i refresh the page it generates a new one. This is inconvenient since i want to be able to identify a student with it.
How can i Make it so it doesnt change?

public string StudentNumber
{


get
{
const int length = 4;
var now = DateTime.Today;
string dobsufix = Dob.ToString().Substring(2, 2);
string enrollsuffix = now.Year.ToString().Substring(2, 2);

var chars = "0123456789";
var random = new Random();
var result = new string(
Enumerable.Repeat(chars, 4)
.Select(s => s[random.Next(s.Length)])
.ToArray());

string stdno = enrollsuffix + dobsufix + result;
return stdno;
}
set { }
}