What is the difference between rj45 and rj11?

What is Registered Jack

RJ, which means Registered Jack, is the acronym that cable connectors usually start with. Connectors using the distinction Registered Jack (RJ) describe a standardized telecommunication network interface. Two of the most common jacks are the RJ45 and RJ11, each with their own specific purpose.

RJ45 VS RJ11

The main difference between these two is their actual use. RJ11 is the cable connector that using in telephone sets while RJ45 is used in networking, where you connect computers or other network elements to each other.
Aside from the application, there are differences in their appearance. The first of which is in the number of cables that are accommodated in each connector. RJ45 has 8 wires inside whereas Rj11 has 4 wires. As a consequence of having to accommodate more wires, RJ45 connectors are also a little bit bigger than RJ11s.

The figure following shows the differences in appearance between RJ11 and rj45.

Figure from What's the difference between RJ11 and RJ45?

In addition, you can plug an RJ11 connector into an RJ45 slot, but you’d better avoid doing this because you might damage the device that has the RJ45 slot, whether it is a switch or a network adapter. This is true when you use an RJ11 connector that is connected to your phone line. The reason behind this is the power that is being delivered by the telephone company into your handset that allows it to function even during a power outage. But with proper knowledge and training, some people have been able to use RJ45s all over their house instead of RJ11s.

Conclusion

1. RJ45 is mostly used with ethernet cables while RJ11 in connecting telephone units.
2. RJ45 has 8 wires inside while Rj11 has 4 wires.
3. RJ45 is bigger in size than RJ11.
4. You cannot plug-in RJ45 cable connector into a RJ11 interface/port/slot however you can do the opposite (you should avoid doing so as it can damage the RJ45 port).