C++: What is the difference between const int*, int* const, and const int* const?

1int x = 10;
2int y = 20;
3
4// const int* — pointer to const
5const int* ptr1 = &x;
6// *ptr1 = 15;  // Error! Cannot modify value
7ptr1 = &y;     // OK — can change pointer
8
9// int* const — const pointer
10int* const ptr2 = &x;
11*ptr2 = 15;     // OK — can modify value
12// ptr2 = &y;  // Error! Cannot change pointer
13
14// const int* const — const pointer to const
15const int* const ptr3 = &x;
16// *ptr3 = 15;  // Error!
17// ptr3 = &y;  // Error!