figurable logical networks, i.e., as a logical-network structure. Let's make an example out of the OSI RM model, by defining another logical network that comprises OSI layers 5 through 7 only (see Fig. 1.27). The two logical networks defined in this way are nested like a Chi- nese box, since logical nodes of network nw2 must exist in logical nodes of net- work nw1. This fact is modelled by hosts relations between nodes of different logical networks, that relates a hosting node to a hosted node. Standardized and manageable layer interface Abstract layer 4 Hosted network: Logical network is nw2= {n21,n22} hosts hosts Logical network is Hosting network: nw1= {n11,n12} Fig. 1.27 The definition of a logical-network structure Obviously, this nesting of logical networks can be repeated until every layer defines a logical network that is either a hosted network only, a hosted network that hosts other networks, or just a hosting network. A logical network that does not host any other network is therefore the counterpart to an OSI application layer. The only logical network that cannot be a hosted network is a network that connects to physical media. Logical networks are models in the N dimension only. By describing a network as a logical-network structure, we actually separate the N dimension into a plane defined by a logical node dimension (NO) and a logical network dimension (NW) (see Fig. 1.28). hosts Fig. 1.28 A logical-network structure is defined in the NO-NW plane. NO=logical node dimen- sion; NW=logical network dimension 1.4 Modeling Network Systems in AMLn The model of a logical network defines logical nodes and connectivity relations only (i.e., routes). However, it says nothing about which layer elements are included in logical nodes. A logical-network structure adds hosts relations, which do not reveal anything about the allocation of layer elements to logical nodes. We can therefore view a logical node as some kind of "package" for layer elements, and a logical-network structure as a specification of how logi