In this paper we present a stochastic network model for packet-switching networks with no buffering capabilities at the nodes. This model can be directly used in the study of all-optical packet-switching (OPS) and optical burst switching (OBS) networks without fiber delay lines (FDLs). Our model provides for the first time a complete description of the dependencies arising among packets from different sources in the network. Such dependencies originate when packets from different sources share a finite number of channels for their transmission through a network link. We relate our model to well-known loss network models for circuit-switching networks and derive expressions for the packet loss probability. We briefly show how our work can be extended to model parallel hybrid optical networks, and propose some promising future lines of work. Our numerical results suggest that the well-known Erlang fixed-point approximation (EFPA) overestimates the blocking probability when compared to our model predictions. They also show that our model is scalable up to network scenarios with at least 30 links with 160 wavelength channels per link. This makes the proposed model an interesting tool for studying the dependencies arising among packets in a realistically-sized OPS/OBS network without FDLs.