Significance The default mode network (DMN) has been suggested to support a variety of internal-state functions in human. Because preclinical models can be used in translational studies of neuropsychiatric disorders, investigations of the DMN in these models may aid the understanding of both physiology and pathophysiology of the human DMN. To our knowledge, this is the first study to investigate the constituents and functional implications of the rat DMN. We provide empirical evidence that the rat DMN is composed of highly connected anatomical and functional subnetworks, which show differential modulation in association with age-related cognitive dysfunction. These findings provide a framework to further explore the physiological basis and behavioral significance of the rodent DMN.