In your example, the correct choice is "that". The extract begins talking about "a network", which could be a connection between as few as two computers. The purpose of that sentence is to define the word "network". The next sentence builds on that definition to introduce the idea of a much wider system of networks, one which connects computers worldwide. Consider the sentence
It is a global system of interconnected computer networks that links devices worldwide.
The meaning of the sentence is
That which connects computers worldwide is a global system of networks.
The referent of the word "it" is not the "network" defined in the previous sentence, but a network "which connects computers worldwide."
If the word "which" is used instead, the word "it" might be taken as referring to the simple "network" previously defined.