RJ45 or RJ11: One Small, One Wide, Both Plastic
The two connectors look alike at a glance and are completely different in size, wiring and purpose. Mixing them is the classic beginner fault.
Quick Answer
| Attribute | RJ45 | RJ11 |
|---|---|---|
| Positions | 8 | 6 |
| Width | Wider | Narrower |
| Contacts used | 8 for Ethernet | 2 or 4 for telephone |
| Typical use | Ethernet networks | Telephone lines, DSL |
| Cable | Cat5e, Cat6, Cat6a | Telephone cable |
| Speeds | Gigabit and above | Voice or DSL rates |
An RJ11 plug fits loosely in an RJ45 socket, which is exactly how damaged contacts happen. Check the plug width before forcing anything.
Identifying Each
Count the positions
Six positions is RJ11 family, eight is RJ45.
Look at the cable
Telephone cable is flat and thin. Ethernet cable is round and thicker.
Check the socket
RJ45 sockets are wider and usually on network equipment.
Read the printing
Ethernet cable is printed with its category, such as Cat6.
What Happens If You Mix Them
- RJ11 into an RJ45 socket: poor contact, possible bent contacts, no connection.
- RJ45 into an RJ11 socket: does not fit.
- Telephone through Ethernet cable: works for DSL, but not for Ethernet.
- Ethernet through telephone cable: fails, the cable lacks the pairs.
Buying Correctly
- Identify what the equipment expects.
- Buy Ethernet cable for networks and telephone cable for lines.
- Match the category to the speed you need.
- Check connectors are moulded with strain relief.
- Test the link before tidying the installation.
Real-World Applications
- Home and office networks.
- VoIP phones, which use Ethernet with RJ45.
- Adapters between the two where DSL modems are involved.
Related Guides
For router wiring, read LAN cable to WiFi router. For fibre links, see fiber optic to Ethernet. For adapters, start with Ethernet cable for MacBook.
FAQ
Can I use telephone cable for Ethernet?
No. It lacks the pairs and the bandwidth Ethernet needs.
Is RJ45 the same as Ethernet?
RJ45 is the connector Ethernet commonly uses over twisted pair cable.
Why does my RJ11 plug feel loose in an RJ45 socket?
It is narrower. Remove it before it damages the contacts.
Which one do I need for broadband?
Depends on the service. DSL arrives on RJ11, fibre and cable modems present Ethernet on RJ45.