\scalebox{0.5}{
\begin{tikzpicture}[
every node/.style={font=\bfseries},
basebox/.style={anchor=west, minimum height=2em, draw, rectangle},
bitbox/.style={basebox, minimum width=2em},
bytebox/.style={basebox, minimum width=16em},
wordbox/.style={basebox, minimum width=32em},
qwordbox/.style={basebox, minimum width=64em},
]

\path
(-9em, 2em) node[basebox, minimum width=5em] {Offsets}
(-9em, 0em) node[basebox, minimum width=5em] {Octet}
(-4em, 2em) node[basebox, minimum width=4em] {Octet}
(-4em, 0em) node[basebox, minimum width=4em] {Bit}
;

\foreach \x in {0,...,3}
	\node[bytebox] at (\x * 16em, 2em) {\scriptsize \x};
\foreach \x in {0,...,31}
	\node[bitbox] at (\x * 2em, 0em) {\scriptsize \x};

\node[basebox, minimum width=4em] at (-4em, -2em) {\scriptsize 0};
\node[basebox, minimum width=4em] at (-4em, -4em) {\scriptsize 32};

\node[basebox, minimum width=5em] at (-9em, -2em) {\scriptsize 0};
\node[basebox, minimum width=5em] at (-9em, -4em) {\scriptsize 4};

\path
( 0 * 2em, -1 * 2em) node[wordbox] {source port}
(16 * 2em, -1 * 2em) node[wordbox] {destination port}
( 0 * 2em, -2 * 2em) node[wordbox] {length}
(16 * 2em, -2 * 2em) node[wordbox] {checksum}
;

\end{tikzpicture}
}