RJ45 vs. RJ11: What's the Difference?

RJ45 vs. RJ11: What's the Difference?

Written by Don Schultz, trueCABLE Senior Technical Advisor, Fluke Networks Copper/Fiber CCTT, BICSI INST1, INSTC, INSTF Certified


If you were born before 1985, you likely still recall old style telephones. Remember those? You went to the store and bought one (hey, it could even be a wireless handset!) and then plugged it into the “phone line” in your home. You likely also recall the short silver satin colored cord with plastic plugs on either end that came with your phone. Those telephone patch cords came with every new phone, kind of like USB charger cables used to come with every USB charger. You know the ones I mean, because you likely have a pile of them in a box somewhere in your garage from all of the phones you bought over the years (and could not bring yourself to toss out). At both ends of those silverish “telephone cords” were RJ11 plugs. Those RJ11 plugs were meant to be plugged into RJ11 jacks and now you could call up your friends and relatives. Then, something started to change. Those plugs and patch cords seemed to get bigger. A lot bigger to the point where you could not plug one of these big plugs into that smaller jack in your house. What happened here? Technology marched ahead, and most landline phones have become a thing of the past. Oh sure, you still see some landlines in use for FAX machines (doctor and lawyer offices are prime examples) and the odd landline telephone but largely they are ancient history.

As networking and telecommunications systems become increasingly intertwined, it's more important than ever to understand the different cable connectors in use. While RJ45 and RJ11 connectors may appear visually similar at first glance, their distinctions have significant implications for ensuring proper connectivity in various applications. Whether installing a new Ethernet infrastructure or servicing legacy telephone wiring, properly identifying these registered jack connectors is critical. In this blog post, we will explore the key differences between RJ45 and RJ11 - examining their physical characteristics, common uses, wiring schematics, compatibility and more. Let's start with defining these connector types and their basic functions.

What are RJ45 and RJ11 Connectors?

RJ45 and RJ11 are types of registered jack (RJ) connectors that have been established as telecommunications industry standards by the Telecommunications Industry Association (TIA).

RJ refers to the standardization of connector designs for linking telephony customer premises equipment to the telephone network wire termination. Both RJ45 and RJ11 are common connector styles, but they serve different purposes in networking and telecommunications applications.

Physical Differences Between RJ45 and RJ11

picture of rj11 plug

RJ11 Modular Connector Plug (6P4C). Photo courtesy of Walmart.


 rj45 connector on white background

RJ45 Modular Connector Plug (8P8C)


While RJ45 and RJ11 connectors may look similar at a glance, they have distinct physical attributes due to their unique design standards. RJ45 connectors contain 8 wires/conductors, arranged in 8 positions within the plastic insulation housing. This is known as 8P8C or eight position/eight contact modular plug.

In contrast, RJ11 connectors handle either two or four conductors, set out across 6 positions inside a smaller connector profile. This is because RJ11 is optimized for basic 2-wire telephone cabling, whereas RJ45 accommodates higher conductor counts suitable for high speed data networking. RJ11 connectors are known as 6P4C or six position/four contact modular plugs. You may also see 6P2C variations.

Due to its increased wire capacity needs, the RJ45 connector has a physically larger size and rectangular shape compared to the compact square design of RJ11. Proper identification and understanding of these physical characteristics is important for ensuring compatibility when working with networking or telephony systems.

Note: RJ12 is the newer variation of RJ11. The difference is RJ11 will make use of up to four conductor wires to handle two pair telephone cable and RJ12 can make use of six conductor wires to handle three pair telephone cable. RJ12 is backward compatible with RJ11, but RJ11 is not forward compatible with RJ12.

Uses of RJ45 and RJ11 Connectors

RJ45 connectors are predominantly used with twisted pair Ethernet cabling for connecting computers, servers, switches, routers and other IP network devices together. Speeds supported range from Fast Ethernet to 10GbE or greater over newer cabling standards like Cat6A.

In contrast, RJ11 is almost exclusively used to terminate basic 2-wire telephone cabling for analog voice services or digital subscriber line (DSL) technologies. Common applications include connecting traditional POTS (plain old telephone system) handsets, analog telephone adapters and integrated DSL modems in devices like routers or home network gateways.

Older telephone systems use variable voltages to modulate your voice patternsThis is known as analog data transmission. On the other end, your voice gets de-modulated into a handset speaker so the other person can hear and understand what you are saying. Even FAX machines use a type of “voice” to translate complex parts of a picture into “bleeps and bloops”. You would have to be another FAX machine on the other end to understand these “bleeps and bloops”, but on the off chance you do understand the bleeps and bloops trueCABLE recommends seeking immediate medical care.

Modern Local Area Networks using RJ45 modular connectors, on the other hand, use data packets. This is known as digital data transmission. The technologies are completely different, although both technologies are still considered “low voltage” communications.

Compatibility and Interchangeability

Due to fundamental differences in their pinout assignments and wiring configurations, RJ11 and RJ45 connectors are strictly incompatible and should never be interchangeably mixed. Attempting to incorrectly join RJ11 cabling to RJ45 ports, or vice versa, risks damaging equipment by potentially connecting wiring pins to incorrect signal channels.

Proper identification and exclusive use of the matching connector type is required for each application to avoid connectivity issues or faults that could undermine network and communication reliability.

The technologies are absolutely incompatible with each other. You can, however, take voice patterns and packetize them for use on a modern Ethernet network. That is called VoIP or Voice over IP. You could even have a “telephone” handset that looks like (but is not) an old style telephone. That is a different blog, though.

Wiring Diagrams and Pinouts

RJ45 connectors follow the T568 wiring standard, either T568A or T568B. This arranges the twisted pairs in an optimal and balanced sequence to reduce crosstalk interference and allow for 100Mbps or higher data rates.

RJ11 connectors use a simplified 2-wire connection scheme. This is called POTS. No twisted pairing or balanced cabling techniques are employed like with structured cabling for data networks.

Shown below are the standard pinout diagrams contrasting the pin assignments between the two connector types:

RJ45 T568 pinout variations chart

RJ45 T568 pinout variations


POTS pinout, 6P6C (RJ12 demonstrated above).  For 6P4C (RJ11), white and blue wires are omitted.  For 6P2C, only red and green conductors are used.

POTS pinout, 6P6C (RJ12 demonstrated above). For 6P4C (RJ11), white and blue wires are omitted. For 6P2C, only red and green conductors are used.


Note that straight pinouts are typically used for FAX machines where reversed pinouts are used for analog telephones. Check your equipment to be sure you know which one to use!

Understanding these pinout distinctions is key, as attempting to interchange connectors risks connecting dissimilar wire pairs to mismatched electrical circuits unintended for their signal types or protocols. Proper termination following documented standards prevents incompatibility issues.

Advantages and Disadvantages

RJ45 connectors provide higher maximum supported speeds well beyond traditional POTS and even digital subscriber line capabilities. Gigabit and 10GbE performance are achievable with quality cabling plants.

However, greater capabilities come at the cost of having a larger connector footprint requiring more materials and often additional labor during installation. Certified channels also command higher component and infrastructure expenses.

RJ11 remains a lower cost, compact solution for legacy telephony applications. But limited to around 1Mbps, it cannot sustain high-bandwidth services such as symmetric broadband internet.

Choice of connector type balances technical needs with associated costs. RJ45 becomes more practical as network demands increase, while RJ11 suffices for simple analog voice applications without advanced data requirements.

Common Questions & Answers

trueCABLE is frequently asked questions about proper 8P8C (aka RJ45) terminations. We have posted the most common ones below.

How can you tell the difference between RJ11 and RJ45 connectors?

The easiest way to visually identify the difference between RJ11 and RJ45 connectors is by counting the number of wires/slots. RJ45 has 8 wire positions arranged in a rectangular shape, while RJ11 only contains 4 wire positions in a more compact square profile. You can also tell by their intended applications - RJ45 is primarily used for Ethernet networking cables between computers and routers, while RJ11 is found terminating telephone lines for analog voice services. Examining the connector size, profiling and wire count are quick ways to distinguish between these common registered jack styles.

Are RJ11 and RJ45 interchangeable?

No, RJ11 and RJ45 connectors are strictly incompatible and should never be interchanged or mixed up. While an RJ11 plug may seemingly fit physically into an RJ45 port, this is not recommended due to their different pinout assignments and wiring configurations. Using the incorrect connector type risks damaging equipment by potentially connecting signal wires to unintended circuits. Their electrical specifications are optimized for separate voice and data applications, so swapping connector types could undermine network reliability and connectivity performance. Proper identification of each registered jack style is required for compatibility.

Can I use a RJ11 to RJ45 for Ethernet?

It is generally not advisable to use an RJ11 connector intended for telephone systems in place of the proper RJ45 connection required for Ethernet networking. While the RJ11 plug may fit mechanically into the RJ45 jack, this configuration would severely limit the link speed and functionality due to the fewer supported wire pairs. RJ11 lacks the wiring positions and pinout defined in TIA standards to realize Gigabit or faster Ethernet speeds as specified by later Cat5e/6 cabling. Sticking to the compatible RJ45 connector type is strongly recommended for optimal wired networking performance and compatibility.

Can RJ11 be used for internet?

In a limited sense, RJ11 is able to carry an internet connection when used with technologies like ADSL that transmit data signals over existing telephone lines. However, direct internet access via high-speed broadband largely requires the superior RJ45 connector type. Wired for only 4-wire voice circuits with limited bandwidth potential, RJ11 is lacking in positions needed to sustain high-performance networking demands. While modems may connect to wall outlets via RJ11, directly linking a computer or router usually necessitates the faster RJ45 widely adopted as the cable plant standard for Ethernet networking capable of Gigabit and faster access speeds.

What is RJ11 used for?

RJ11 connectors are commonly used to terminate basic 2-wire telephone cables for analog voice services. Typical applications involve connecting traditional landline phones to wall jacks installed in buildings. RJ11 is also frequently used to link integrated DSL modems found in wireless routers and gateways to the telephone outlet for transmitting internet data signals over existing voice lines. Due to its smaller size and 4-position design optimized for a single telephone circuit, RJ11 remains suitable for basic telephony applications that do not require advanced multi-wire networking capabilities.

What happens if you plug RJ11 into RJ45?

While an RJ11 connector may seem to fit physically into an RJ45 jack, plugging the incorrect type can potentially damage the port or connected equipment. This is because RJ11 and RJ45 are wired differently according to incompatible pinout standards. Signaling and power may end up connected to mismatched conductor pairs not intended for those signals. At best, the link simply will not function as expected. But significant faults could arise due to the electrical mismatch. To avoid any reliability or performance issues, it is always safest to exclusively use RJ11 only with telephone systems and RJ45 solely for Ethernet networking applications as designed and specified.

What is RJ45 used for?

RJ45 connectors are nearly universally adopted as the standard interface for physical Ethernet connections. They are commonly used for linking wired Ethernet devices like computers, switches, routers, IP phones and more within local area networks. RJ45 enables transmission of digital data and is capable of supporting internet access speeds from Fast Ethernet to 10 Gigabit Ethernet or higher depending on the cabling type. As Ethernet networking gains prevalence in both commercial and residential settings, RJ45 has emerged as the preeminent cable connector facilitating IP communications and connectivity.

Why might you use a RJ11 connector?

While RJ11 has limited suitability for modern high-speed data transmission needs, there remain scenarios where this legacy registered jack style proves useful. RJ11 continues serving basic analog telephone systems effectively and economically. It is commonly employed for infrastructure wiring within buildings to terminate wall jacks for traditional landline phones. RJ11 connectors also allow connecting integrated DSL modems in devices like routers and gateways to telephone outlets, transmitting internet data over existing voice lines when no Ethernet backbone is available. For these specialized applications not requiring robust networking capabilities, RJ11 connectors endure as a practical backwards-compatible solution.


In summary, RJ45 and RJ11 connectors each have their appropriate uses depending on the communication technology and performance needs. RJ45 has become the ubiquitous standard for modern Ethernet networking given its higher speeds and evolving cabling standards. However, RJ11 continues serving basic telephone systems effectively and economically.

The key is understanding the inherent differences between these connectors - from their physical attributes and pinout wiring, to incompatible specifications preventing interchangeability. With telecommunications converging over IP, RJ45 usage will likely expand to new scenarios that combine voice, data and video. Yet RJ11 endures as a fit-for-purpose solution for legacy analog telephony applications without advanced networking demands.

Proper identification of RJ45 versus RJ11 connectors and exclusive use of the matching type is essential for ensuring systems function as intended without degradation or faults. By comprehending their distinctions, network engineers, installers and system owners can confidently deploy the appropriate solution according to connectivity requirements and performance objectives.




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