% A complete graph
% Author: Quintin Jean-Noël
% <http://moais.imag.fr/membres/jean-noel.quintin/>
\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage[nomessages]{fp}% http://ctan.org/pkg/fp
\usepackage{tikz}
\usetikzlibrary[topaths]


\begin{document}
    \begin{tikzpicture}
        \node[vertex] (N1) at (0,0) {};
        \node[vertex] (N2) at (2,0) {};
        \node[vertex] (N3) at (4,0) {};
        \node[vertex] (N4) at (1,1) {};
        \node[vertex] (N5) at (3,1) {};
        \node[vertex] (N6) at (2,2) {};

        \draw (N1) -- (N4) -- (N2) -- (N5) -- (N4) -- (N6) -- (N5) -- (N3) -- (N2) -- (N1);
    \end{tikzpicture}
\end{document}