The invention relates to a method and a gateway (N1) for transmitting data from a first router (R1) to a second router (R2) via an external communications network (ATM), which makes it possible to couple notably local networks (LAN1, LAN2) connected to these routers. The first router (R1) is connected to the communications network (ATM) via a gateway (N1) which, among other things, is used for conversion between a transmission protocol used by the first router (R1) and a transmission protocol used by the communications network (ATM). The first router assigns to the data a hardware address (MAC2) which identifies the second router (R2) and constitutes a target address. In the gateway (N1) a network address (ATM2) which identifies an output point (N2) from the communications network is assigned to this target address. On the basis of this network address the data are transmitted to the output point (N2) of the communications network and from there to the second router (R2).