  The calculated quasiparticle band structure of bulk hexagonal boron nitride
using the all-electron GW approximation shows that this compound is an
indirect-band-gap semiconductor. The solution of the Bethe-Salpeter equation
for the electron-hole two-particle Green function has been used to compute its
optical spectra and the results are found in excellent agreement with available
experimental data. A detailed analysis is made for the excitonic structures
within the band gap and found that the excitons belong to the Frenkel class and
are tightly confined within the layers. The calculated exciton binding energy
is much larger than that obtained by Watanabe {\it et al} using a Wannier model
to interpret their experimental results and assuming that h-BN is a
direct-band-gap semiconductor.
