I am sure this has been asked, but I am having trouble understanding how to bit mask a 8 bit register.

Suppose I have a 8 bit register. The bits are set via `PORTD`. I want to change all the bits except the 2 LSB.

```
PORTD |= 0b111011XX;

```

XX are the 2 LSB I don't want to change.

How do I do this?