Many interacting complex systems in biology, physics, technology and social systems can be represented in a form of large networks. The networks are mathematically represented by graphs. A graph is usually represented by adjacency or Laplacian matrix. Many important features of the underlying structure and dynamics of them can be extracted from the analysis of the spectrum of graphs. Spectral analysis of the so called normalized Laplacian matrix of large networks has become popular in recent years. The Laplacian matrices of empirical networks are in form of unstructured large sparse matrices.