\multicolumn を使う.
\begin{table} \caption{A,B,C} \label{tbl:packet_type} \begin{center} \begin{tabular}{c|c|c} & \multicolumn{2}{c}{AAA} \\ \cline{2-3} & A & B \\ \hline C & AC & BC \\ D & AD & BD \\ \end{tabular} \end{center} \end{table}
\multicolumn を使う.
\begin{table} \caption{A,B,C} \label{tbl:packet_type} \begin{center} \begin{tabular}{c|c|c} & \multicolumn{2}{c}{AAA} \\ \cline{2-3} & A & B \\ \hline C & AC & BC \\ D & AD & BD \\ \end{tabular} \end{center} \end{table}