Let's first define a fully connected network. We call a graph where every vertex is directly connected to every other vertex a fully connected network. 

If we have a fully connected network, N, then the number of edges in the network can be represented by the following:

Now, this is where density comes into play. Density measures the number of observed edges to the maximum number of edges, if Edges_Observed is the number of edges we want to observe. It can be framed as follows:

Note that for a triangle, the density of the network is 1 and this represents the highest possible connected network.