Creating your own Tinyurl style uid

static Random _random = new Random();

public static void main()
{
    // Size of the key, 6
    HashSet<string> set = new HashSet<string>();
    int clashes = 0;
    for (int n=0;n < 1000000;n++)
    {
    	StringBuilder builder = new StringBuilder();

    	for (int i =0;i < 7;i++)
    	{
    		if (_random.NextDouble() > 0.5)
    		{
    			builder.Append((char)_random.Next(97,123));
    		}
    		else
    		{
    			builder.Append(_random.Next(0,9).ToString());
    		}
    	}

    	if (set.Contains(builder.ToString()))
    	{
    		clashes++;
    		Console.WriteLine("clash: (" +n+ ")" +builder.ToString());
    	}

    	set.Add(builder.ToString());
    	_random.Next();
    	//Console.Write(builder.ToString());
    }

    Console.WriteLine("Clashes: " +clashes);
    Console.ReadLine();
}

p = ( 0.5 * ( (0.5*1/10) + (0.5*1/26) ) )^6

Id  URL
 1  http://google.com
 2  ...
... ...
156 ...
... ...

http://site.com/1
http://site.com/2
...
http://site.com/2W
...

select dbo.FX_CONV (123456) -- returns "1f5n"

select dbo.FX_CONV (123457) -- returns "1f5o"

7QDBkvCA1+B9K/U0vrQx1A
7QDBkvCA1+B9K/U0vrQx1A==

5:$Hj:Pf4RLB9%kULj