Interior volume within the horizon of a black hole is a non-trivial concept
which turns out to be very important to explain several issues in the context
of quantum nature of black hole. Here we show that the entropy, contained by
the {\it maximum} interior volume for massless modes, is proportional to the
Bekenstein-Hawking expression. The proportionality constant is less than unity
implying the horizon bears maximum entropy than that by the interior. The
derivation is very systematic and free of any ambiguity. To do so the precise
value of the energy of the modes, living in the interior, is derived by
constraint analysis. Finally, the implications of the result are discussed.Comment: Two new references and additional discussions added, to appear in
Phys. Lett.
