  We consider Laplacians on periodic metric graphs with unit-length edges. The
spectrum of these operators consists of an absolutely continuous part (which is
a union of an infinite number of non-degenerated spectral bands) plus an
infinite number of flat bands, i.e., eigenvalues of infinite multiplicity. Our
main result is a localization of spectral bands in terms of eigenvalues of
Dirichlet and Neumann operators on a fundamental domain of the periodic graph.
The proof is based on the spectral band localization for discrete Laplacians
and on the relation between the spectra of discrete and metric Laplacians.
