RJ11 ⇔ RJ45
The telephone cable that arrives at the subscriber's home is called RJ11, Registered Jack 11, it has 4 conductors grouped into 2 colored pairs. The socket consists of 6 physical positions and 4 electrical contacts of which only 2 are used (6P2C).
These two central contacts are used for the telephone line.
RJ45 Registered Jack 45 has 8 positions and 8 electrical contacts (8P8C), this connector is commonly used for Ethernet connections, i.e. networks, especially to connect computers to the Internet.
The right RJ45 cable is used to connect a computer to a Hub or Switch. The cross-wire RJ45 cable is usually used to connect two Hubs or two Switches or two computers.