arrow to multipart rectangle

Question

I want to do something like that:

But I'm having a hard time with the arrow. I was wondering if there was a way to do it without hardcoding every coordinates.

Here is the code of the multipart rectangle:

\documentclass[]{article}
\usepackage{tikz}
\usepackage[skip=2pt]{caption}
\usetikzlibrary{positioning,decorations.pathreplacing, shapes, arrows}
\settowidth{\textwidth}{Multipart rectangles with no line shortening}
\begin{document}

\begin{tikzpicture}[auto, node distance=1cm,
rect/.style={
    rectangle split,
    rectangle split parts=3,
    rectangle split horizontal,
    rectangle split part fill={green!50,green!50,gray!50},
    draw=black,
    rounded corners,
    text width = 3cm
}]

% Split Rectangle
\node[rect] {
    \centerline{ether}
        \nodepart  {two} \centerline{ip}
        \nodepart[text width=5cm]{three}
};

\end{tikzpicture}
\end{document}

Answer 1

As described in the manual, section 70.6 Shapes with multiple text parts, there are predefined anchors at the borders between the node parts. If you name your node foo, then foo.two split north is the point where your arrow is pointing to, so you can do

\draw [<-] (foo.two split north) -- +(0,1cm) node[above]{bar};

to get

\documentclass[]{article}
\usepackage{tikz}
\usepackage[skip=2pt]{caption}
\usetikzlibrary{positioning,decorations.pathreplacing, shapes, arrows}
\settowidth{\textwidth}{Multipart rectangles with no line shortening}
\begin{document}

\begin{tikzpicture}[auto, node distance=1cm,
rect/.style={
    rectangle split,
    rectangle split parts=3,
    rectangle split horizontal,
    rectangle split part fill={green!50,green!50,gray!50},
    draw=black,
    rounded corners,
    text width = 3cm,
    align=center % <- alternative to \centerline everywhere
}]

% Split Rectangle
\node[rect] (foo) {
    ether
    \nodepart  {two} ip
    \nodepart[text width=5cm]{three}
};

\draw [<-] (foo.two split north) -- +(0,1cm) node[above]{bar};
\end{tikzpicture}
\end{document}