8. Use the digits $1,2,3,4,5,6$ to form a six-digit number $\overline{a b c d e f}$, where different letters represent different digits from 1 to 6. It is required that the two-digit number $\overline{a b}$ is a multiple of 2; and $\overline{a b c}$ is a multiple of 3; $\overline{a b c d}$ is a multiple of 4; $\overline{a b c d e}$ is a multiple of 5; $\overline{a b c d e f}$ is a multiple of 6. Find all such six-digit numbers and explain your reasoning process.
8. Since $\overline{a b}$ is a multiple of 2, $\overline{a b c d}$ is a multiple of 4, and $\overline{a b c d e f}$ is a multiple of 6, we have
$$
\begin{array}{l}
b, d, f \in\{2,4,6\}, \\
a, c, e \in\{1,3,5\} .
\end{array}
$$

Since $\overline{a b c d e}$ is a multiple of 5, we have $e=5$.
At this point, the 1st digit $a$ and the 3rd digit $c$ can only be 1 or 3, so $a+c=4$.
Since $\overline{a b c}$ is a multiple of 3, we have
$3 \mid a+b+c=4+b$.
Thus, we can only have $b=2$.
At this point, the desired six-digit number can only be found among the following numbers:
123456, 123654, $321456,321654$.
Upon inspection, by $4 \times 1234,4 \times 3214$,
we find that only 123654 and 321654 are the six-digit numbers that satisfy the problem's requirements.