The problem of network coding for multicasting a single source to multiple sinks has first been studied by Ahlswede, Cai, Li and Yeung in 2000, in which they have established the celebrated maxflow mini-cut theorem on non-physical information flow over a network of independent channels. On the other hand, in 1980, Han has studied the case with correlated multiple sources and a single sink from the viewpoint of polymatroidal functions in which a necessary and sufficient condition has been demonstrated for reliable transmission over the network. This paper presents an attempt to unify both cases, which leads to establish a necessary and sufficient condition for reliable transmission over a noisy network for multicasting all the correlated multiple sources to all the multiple sinks. Furthermore, we address also the problem of transmitting "independent" sources over a multiple-access-type of network as well as over a broadcast-type of network, which reveals that the (co-) polymatroidal structures are intrinsically involved in these types of network coding. Several network models with correlated multiple sources have been studied by some people, e.g., by Barros and Servetto [10], Ho, Médard, Effros and Koetter [14], Ho, Médard, Koetter, Karger, Effros, Shi and Leong [15], Ramamoorthy, Jain, Chou and Effros [16]. Among others, [14], [15] and [16] consider (without attention to the converse part) error-free network coding for two (or possibly multiple) stationary memoryless correlated sources with a single (or possibly multiple) sink(s) to study the error exponent problem, where we notice that all the arguments in [14], [15] and [16] can be validated only within the class of stationary memoryless sources of integer bit rates and error-free channels (i.e., the identity mappings) all with one bit capacity (or integer bits capacity, which is allowed by introducing multiple edges); these restrictions are needed solely to invoke "Menger's theorem" in graph theory. The main result in the present paper is quite free from such seemingly severe restrictions, because we can dispense with the use of Menger's theorem.On the other hand, [10] revisits the same model as in Han [3], while [16] mainly focuses on the network with two correlated sources and two sinks to discuss the separation problem of distributed source coding and network coding, where, in addition, cases of two-sources three-sinks, and three-