Multicast data transfer

In response to a file request from a host 3a, a server 7 retrieves the file from a content provider 2. A group comprising the host 3a is defined and the file sent to the group as a sequence of data packets A-G. Any further hosts 3b, 3c in the same locational area as host 3a that submit a request for the same file during the transmission of the data packet sequence are added to the group and receive the remaining packets in the sequence. At the end of the sequence, the server 7 re-transmits the data packet sequence to hosts 3b, 3c so that they receive the data packets dropped or missed in the original transmission. A host 3b that moves to another location during the transmission submits a NACK message, which is treated by a server 7′ associated with the second locational area as a file request.