(4 billion and change being the number of combinations that can be made with IPv4's 32 address bits).

This may be a design decision if there are more address bits available than are necessary to support the installed memory device(s).

Only a 5-bit opcode was used in order to simplify the basic logic and gain an address bit.

Here's a nice twist on the old charge of address bit.

For a load, the compiler would need to rotate the obtained value according to the original low 2 address bits.

It's just a lot more address bits is essentially all that happens.

The upper address bits are the (virtual) page number.

A 0 indicates that the address bits must be considered (exact match); a 1 in the mask is a "don't care".

If a slave has all the address bits so far, it returns a 0.

The master uses this simple behavior to search systematically for valid sequences of address bits.