Example: P2P system topology

% P2P system topology
% Author Fiandrino Claudio 2011 - http://claudiofiandrino.altervista.org/
% Description: physical and overlay topology characterization
\documentclass{article}
\usepackage{tikz}
\usepackage[active,tightpage]{preview}
\usetikzlibrary{shapes}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{7pt}
\definecolor{pinegreen}{cmyk}{0.92,0,0.59,0.25}
\definecolor{royalblue}{cmyk}{1,0.50,0,0}
\definecolor{lavander}{cmyk}{0,0.48,0,0}
\definecolor{violet}{cmyk}{0.79,0.88,0,0}
\tikzstyle{cblue}=[circle, draw, thin,fill=cyan!20, scale=0.8]
\tikzstyle{qgre}=[rectangle, draw, thin,fill=green!20, scale=0.8]
\tikzstyle{rpath}=[ultra thick, red, opacity=0.4]
\tikzstyle{legend_isps}=[rectangle, rounded corners, thin, 
                       fill=gray!20, text=blue, draw]
                        
\tikzstyle{legend_overlay}=[rectangle, rounded corners, thin,
                           top color= white,bottom color=green!25,
                           minimum width=2.5cm, minimum height=0.8cm,
                           pinegreen]
\tikzstyle{legend_phytop}=[rectangle, rounded corners, thin,
                          top color= white,bottom color=cyan!25,
                          minimum width=2.5cm, minimum height=0.8cm,
                          royalblue]
\tikzstyle{legend_general}=[rectangle, rounded corners, thin,
                          top color= white,bottom color=lavander!25,
                          minimum width=2.5cm, minimum height=0.8cm,
                          violet]
\begin{document}
\begin{tikzpicture}[auto, thick]
 % Cloud creation
  \node[cloud, fill=gray!20, cloud puffs=16, cloud puff arc= 100,
        minimum width=7cm, minimum height=2.5cm, aspect=1] at (0,0) {};

 % Physical layer nodes
  \foreach \place/\x in {{(-2.5,0.3)/1}, {(-1.75,-0.55)/2},{(-1.2,0.55)/3},
    {(-0.75,-0.7)/4}, {(-0.25,0)/5}, {(0.25,0.7)/6}, {(0.75,-0.3)/7}, 
    {(1.5,0)/8},{(2.5,0.4)/9}}
  \node[cblue] (a\x) at \place {};
 
 % Physical layer links
  \path[thin] (a1) edge (a2);
  \path[thin] (a1) edge (a3);
  \path[thin] (a2) edge (a3);
  \path[thin] (a3) edge (a6);
  \path[thin] (a2) edge (a4);
  \path[thin] (a5) edge (a6);
  \path[thin] (a5) edge (a4);
  \path[thin] (a5) edge (a2);
  \path[thin] (a5) edge (a7);
  \path[thin] (a6) edge (a7);
  \path[thin] (a6) edge (a9);
  \path[thin] (a6) edge (a8);
  \path[thin] (a8) edge (a9);
  \path[thin] (a7) edge (a8);
 
 % ISPs labelling
  \draw[very thick, blue, dashed] 
    (1.8,1.2).. controls +(left:2.2cm) and +(down:0.5cm) ..(0.7,-1.1);
  \draw[very thick, blue, dashed] 
    (-3,1).. controls +(right:2.2cm) and +(down:0.4cm) ..(-1,-1);
  \node[legend_isps] (i1) at (-2.75,-2) {\textsc{Isp 1}};
  \node[legend_isps] (i2) at (0,-2) {\textsc{Isp 2}};
  \node[legend_isps] (i3) at (2,-2) {\textsc{Isp 3}};
  \draw[-latex, thick, red] (i1) -- (-2.75,0);
  \draw[-latex, thick, red] (i2) -- (0,-0.7);
  \draw[-latex, thick, red] (i3) -- (2,0);

 % Overlay layer nodes
  \foreach \place/\i in {{(-2.5,2.3)/1},{(-1.75,1.45)/2},{(-1.2,2.55)/3},
    {(-0.75,1.3)/4}, {(-0.25,2)/5}, {(0.25,2.7)/6}, {(0.75,1.7)/7},
    {(1.5,2)/8},{(2.5,2.4)/9}}
    \node[qgre] (b\i) at \place {};
 
 % Overlay layer links
  \path[thin] (b1) edge (b2);
  \path[thin] (b2) edge (b4);
  \path[thin] (b4) edge (b5);
  \path[thin] (b5) edge (b7);
  \path[thin] (b4) edge (b3);
  \path[thin] (b7) edge (b8);
  \path[thin] (b8) edge (b9);
  \path[thin] (b9) edge (b6);
  \path[thin] (b6) edge (b3);
  \path[thin] (b3) edge (b1);
  \path[thin] (b5) edge (b8);
  \path[thin] (b5) edge (b3);
  \path[thin] (b5) edge (b6);
 
 % Links between overlay and physical topology
  \foreach \i in {1,...,9}
    \path[rpath] (a\i) edge (b\i);
 
 % Legends
  \node[legend_general] at (0,4){\textsc{P2P System Topology}};
  \node[legend_overlay] at (6,2){\textsc{Overlay Network}};
  \node[legend_phytop] at (6,0){\textsc{Physical Network}};
\end{tikzpicture}
\end{document}