The network to analyze must be directed, otherwise there are no special constraints.
The algorithm is used to disclose affinities/diversities between neighboring nodes. Many properties of networks and of processes that take place on networks are affected by the presence of degree-degree correlations.
The algorithm was implemented and documented by S. Fortunato, integrated by S. Fortunato and W. Huang.
Pastor-Satorras, R., Vazquez, A., Vespignani, A. (2001). Dynamical and Correlation Properties of the Internet. Physical Review Letters 87:258701.