RJ45 (Registered Jack 45) is a physical interface often used for terminating twisted pair type cables. RJ stands for Registered pack which is part of theUnited states Code of Federal Regulations. It has eight pins or electrical connections per connector.
It is frequently terminated using the T568A or T568B pin/pair assignments that are defined in TIA/EIA-568-B:
RJ stand for the registered jack and the mean of the 45 is that the pin number of jack 1236 are only active other pin are deactive pin there is a break of activation at 45 pin so its known as the rj45.