Given an irreducible local conformal net A of von Neumann algebras on the circle and a finite-index conformal subnet B of A, we show that A is completely rational iff B is completely rational. In particular this extends a result of F. Xu for the orbifold construction. By applying previous results of Xu, many coset models turn out to be completely rational and the structure results in [KLM] hold. Our proofs are based on an analysis of the net inclusion B in A; among other things we show that, for a fixed interval I, every von Neumann algebra R intermediate between B(I) and A(I) comes from an intermediate conformal net L between B and A with L(I)=R. We make use of a theorem of Watatani (type II case) and Teruya and Watatani (type III case) on the finiteness of the set I(N,M) of intermediate subfactors in an irreducible inclusion of factors N in M with finite Jones index [M:N]. We provide a unified proof of this result that gives in particular an explicit bound for the cardinality of I(N,M) which depends only on [M:N].