In this article the authors examine the concept of software defined network (SDN). In the beginning there is a short historical background of the ”software defined network” as the scientific and technological concept given, by whom it was introduced and what it means. Substantial attention is paid to the OpenFlow protocol. Further, the authors consider development of SDN technology similarly to ideas suggested in MPLS concept, discuss the issues of distributed controllers using ONIX, Kandoo as examples and describe the mechanism of interaction between applications and SDN controller, which they have devised.