The fundamental equation of micromagnetics is analytically solved under the following assumptions: the direction of the magnetization vector depends only on one coordinate, the anisotropy energy is uniaxial and of the second degree, the magnetization lies in a plane normal to this coordinate and including the easy axis, the external magnetic field is parallel to the easy direction. One class of periodic solutions is discussed in detail, corresponding to fully magnetized domains divided from each other by means of Bloch walls; for this class every pair of values of the field and of the period selects one and only one solution. Under the consistent assumption that the period is constant during slow changes of the external field, the magnetization curves for different periods are determined. For domains much larger than one ``classical'' Bloch wall, it is possible to define a wall energy per unit area which differs from the ``classical'' formula because of a factor (1+h)½(his the ratio between the external field and the anisotropy one). The magnetization vector within the walls points alternatively in opposite directions; solutions with magnetization vector within all the walls pointing toward the same direction belong to another class and are unstable.