Computational grids are probably among the most heterogeneous computing systems. However, they are very attractive for their potential computational power. Their heterogeneity is especially perceptible during the deployment of grid applications. In particular, an automatic and efficient deployment of grid applications on resources requires to have access to the characteristics of the resources. This paper presents a description model of (grid) networks which provides a functional view of the network topology. The simplicity of the proposed model does not hinder the description of complex network topologies (asymmetric links, firewalls, non-IP networks, non-hierarchical topologies).