We formulate a frequency-division multiple access (FDMA) networking problem for wireless mobile ad-hoc networks (MANETS) to jointly optimize end-to-end user rates, routes, link capacities, transmitted power, frequency and power allocation across subcarriers and fading states. We show that the resulting non-convex optimization problem has zero duality gap. For some types of FDMA networks this result is exploited to reformulate the original problem into a (computationally tractable) convex optimization problem. We further exploit the lack of duality gap to show that conventional layering can be optimal in FDMA wireless MANETS. Specifically, if we select Lagrange multipliers appropriately, we can decompose the original problem in smaller sub-problems associated with the conventional networking layers. The solution of these per-layer optimization problems coincides with the solution of the originally formulated cross-layer optimization problem.