I know that a PC can have multiple network adapters each of which with different network identifiers (IP,MAC etc.). 

My question is, can a certain PC have more than one active default gateway? And if so, how does the PC (or better say the OS) knows to what adapter use for any given packet?


Question: Is this function bijective?
Answer: Hint $\ $ Glue the bijections $\rm\ b = f_{\,0}(b):\Bbb N\to \Bbb N\ $ and $\rm\ 1\!-\!b = f_{\!\ 1}(b): \Bbb N\to\, {-}\Bbb N\cup \{0\}\ $ as below:

If $\rm\:S_0,\,S_1\:$ are disjoint subsets of $\,\Bbb Z\,$ and $\rm\:f_{\,i}(b) :\Bbb N\to S_i\,$  are bijections for $\rm\,i\in\{0,1\}\,$ then $\rm\:f(a,b):\{0,1\}\times\Bbb N \to S_0\!\cup S_1\,$ is a bijection for  $\rm\ f(a,b)\ =\ a\,f_{\!\ 1}(b) + (1\!-\!a)\,f_{\,0}(b)$