\documentclass[convert=true, tikz]{standalone}

\begin{document}
\begin{tikzpicture}
  \node (1) at (0,0) [shape=circle, draw] {1};
  \node (2) at (2,0) [shape=circle, draw] {2};
  \node (3) at (4,0) [shape=circle, draw] {3};
  \node (4) at (0,2) [shape=circle, draw] {4};
  \node (5) at (2,2) [shape=circle, draw] {5};
  \node (6) at (4,2) [shape=circle, draw] {6};

  \draw (2) -- (3) -- (6) -- (5) -- (2) -- (6);
  \draw (3) -- (5);

  \draw (1) -- (2) -- (4) -- (1);

  \draw (4) -- (5);
\end{tikzpicture}
\end{document}