ATmega168 datasheet (375 pages of ATmega information overload) page 3 says
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port B output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up
resistors are activated.
Ports C and D (i.e. all the general purpose I/O pins) are likewise endowed.
-j