  We use the entanglement negativity, a measure of entanglement for mixed
states, to probe the structure of entanglement in the ground state of a
topologically ordered system. Through analytical calculations of the negativity
in the ground state(s) of the toric code model, we explicitly show that the
entanglement of a region $A$ and its complement $B$ is the sum of two types of
contributions. The first type of contributions consists of \textit{boundary
entanglement}, which we see to be insensitive to tracing out the interior of
$A$ and $B$. It therefore entangles only degrees of freedom in $A$ and $B$ that
are close to their common boundary. As it is well-known, each boundary
contribution is proportional to the size of the relevant boundary separating
$A$ and $B$ and it includes an additive, universal correction. The second
contribution appears only when $A$ and $B$ are non-contractible regions (e.g.
on a torus) and it consists of long-range entanglement, which we see to be
destroyed when tracing out a non-contractible region in the interior of $A$ or
$B$. Only the long-range contribution to the entanglement may depend on the
specific ground state under consideration.
