Why do these two number comparisions not work the same way?

Question

Integer i3 = 10;
Integer i4 = 10;
System.out.println(i3 == i4);
Integer i5 = 1000;
Integer i6 = 1000;
System.out.println(i5 == i6);

I got output for the above code as,

true
false

I can understand how i3==i4, but what is the reason to i5==i6 become false.