1. An apparatus for transmitting data, the apparatus comprising: a management unit that manages network-topology information of clients participating in peer-to-peer (P2P) service; an interface unit that provides to a first client a peer list including information of a plurality of clients having a minimum network distance to the first client, based on the network topology information, when receiving a request for content from the first client; and a peer-list-generation unit that generates the peer list, wherein the interface unit provides the peer list to the first client in the case where a number of clients included in the peer list is the same as the predetermined value, wherein the peer-list-generation unit determines whether a number of clients within the peer list is identical to a predetermined value, wherein the peer-list-generation unit adds information of a second client to the peer list in a case where there exists the second client sharing the content in the at least one second autonomous network having the minimum network distance to a first autonomous network to which the first client belongs, wherein the peer-list-generation unit adds to the peer list information of a third client sharing the content in at least one third autonomous network having a second-shortest network distance to the first autonomous network next to the at least one second autonomous network in the case where a number of clients included in the peer list is less than the predetermined value.