One of the most important problems in biology is to discover the relationship between topological properties and functional features of proteins in Protein-Protein Interaction (PPI) network. The centrality-lethality rule reveals that hub proteins tend to be more essential than non-hub proteins. Why hub proteins tend to be more essential? Here, we classified hub proteins into two types: essential hub and nonessential hub. We found that essential hubs are more important to the survival of the cell and they can be called intra-modular hubs while nonessential hubs tend to be inter-modular hubs.