This disclosure relates to transmitting data packets from a source to a destination within a communications network. A data packet is received from the source located in a local sub-network of the network. The data packet includes a first network layer protocol header having a source address containing the local sub-network address of the source, a destination address of the destination, a first field indicating a length of the source address and a second field indicating a length of the destination address. The first network layer protocol header is transformed by modifying the source address and the first field indicating the length of the source address, such that the modifying includes appending to the local sub-network address a prefix of the sub-network to make the source address an address of a higher-level network. The data packet is then forwarded toward the destination in the higher-level network.