How can I draw a very thin row?

\documentclass{article}
\usepackage{amsmath,array}
\providecommand{\textsubscript}[1]{$_\textnormal{#1}$}

\begin{document}
\setlength{\extrarowheight}{2pt}
\begin{tabular}{|*6{c|}}
\multicolumn{1}{c}{1 bit}  &
\multicolumn{1}{c}{2 bits} &
\multicolumn{1}{c}{1 bit}  &
\multicolumn{1}{c}{4 bits} &
\multicolumn{1}{c}{4 bits} &
\multicolumn{1}{c}{4 bits} \\
\hline
VALIDEZ & DIVERSIFICACI\'ON &
TAMA\~NO & KDC\textsubscript{AUTH/ENC} &
KDC\textsubscript{KEK} & KDC\textsubscript{MAC} \\
\hline
\multicolumn{6}{@{}c@{}}{\vrule height 2pt depth 2pt \hrulefill \vrule height 2pt depth 2pt} \\
\multicolumn{6}{c}{2 bytes}
\end{tabular}
\end{document}

A more general solution that works also when colortbl is loaded (via \usepackage[table]{xcolor}, in your case)

\documentclass{article}
\usepackage{amsmath,array}
\usepackage[table]{xcolor} % Need for colour cells

\providecommand{\textsubscript}[1]{$_\textnormal{#1}$}

\newcommand{\range}[1]{%
  \multicolumn{#1}{@{}c@{}}{%
    \vrule height 2.2pt depth 1.8pt
    \leaders\hrule\hskip 0pt plus 1filll % note the three l's
    \vrule height 2.2pt depth 1.8pt
  }%
}

\begin{document}
\setlength{\extrarowheight}{2pt}
\begin{tabular}{|*6{c|}}
\multicolumn{1}{c}{1 bit}  &
\multicolumn{1}{c}{2 bits} &
\multicolumn{1}{c}{1 bit}  &
\multicolumn{1}{c}{4 bits} &
\multicolumn{1}{c}{4 bits} &
\multicolumn{1}{c}{4 bits} \\
\hline
VALIDEZ & DIVERSIFICACI\'ON &
TAMA\~NO & KDC\textsubscript{AUTH/ENC} &
KDC\textsubscript{KEK} & KDC\textsubscript{MAC} \\
\hline
\range{6}\\
\multicolumn{6}{c}{2 bytes}
\end{tabular}
\end{document}

You can use \range{<number>} for spanning any number of columns; six, in this case.