Efficient message distribution to subsets of computer networks

A computer sends a message to each of a number of recipient computers of a computer network by sending the message as a multicast message to near ones of the recipient computers and sending the message as unicast messages to far ones of the recipient computers. The sending computer determines the circumstances under which a combination of multicast and unicast messages are efficient by determining that many recipient computers are near the sending computer and that few recipient computers are far. The sending computer makes such a determination by determining no more than a predetermined number of recipient computers are at least a predetermined distance further from the sending computer than are the others of the recipient messages. The sending computer can also determine that the burden imposed upon the computer network by a multicast message is justified by the need to deliver the message to its intended recipients. For intended recipients which are too far and too few to justify use of a multicast message, unicast messages are sent.