  We carry out a systematic study of the exact block entanglement in XXZ
spin-chain at Delta=-1/2. We present, the first analytic expressions for
reduced density matrices of n spins in a chain of length L (for n<=6 and
arbitrary but odd L) of a truly interacting model. The entanglement entropy,
the moments of the reduced density matrix, and its spectrum are then easily
derived. We explicitely construct the "entanglement Hamiltonian" as the
logarithm of this matrix. Exploiting the degeneracy of the ground-state, we
find the scaling behavior of entanglement of the zero-temperature mixed state.
