For an ordinary Ethernet cable, “RJ45” is familiar shorthand. The more precise name for the physical plug is an 8P8C modular connector: eight positions and eight contacts. The distinction matters because “RJ” means “Registered Jack,” a label for an interface and its wiring arrangement—not simply a connector’s shape.
Why people call an Ethernet plug RJ45
In everyday networking, “RJ45” is readily understood to mean the modular plug used on standard Ethernet cables. The term is convenient, but it blurs two different things: the physical connector and the interface or wiring configuration associated with a registered-jack designation.
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“RJ” stands for “Registered Jack.” A registered-jack label identifies an interface configuration, not just the number of slots in a connector. An eight-position connector can be wired for different configurations, so its appearance alone does not establish which one it uses. NECABLES’ overview of RJ and 8P8C terminology describes this distinction and notes configurations including RJ61C, RJ48C, and keyed RJ45S.
What 8P8C describes—and what it does not
In 8P8C, “8P” means eight positions and “8C” means eight contacts. It describes the connector’s physical arrangement. It does not, by itself, identify Ethernet, specify a cable category, or guarantee a performance level. Two connectors with the same general form can have different wiring or serve different interfaces.
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For Ethernet, the cable’s pair termination matters as well as the plug shape. The KINGCONN wiring guide describes the T568A and T568B pair assignments and notes that 10/100BASE-T uses two pairs while gigabit Ethernet uses all four. Those details concern how a cable is terminated and used; “8P8C” alone does not tell you its wiring.
How RJ45S and RJ48 show why the names differ
| Term | What the term identifies | Why the distinction matters |
|---|---|---|
| 8P8C | A modular connector with eight positions and eight contacts. | Describes physical form, not a specific interface or wiring scheme. |
| RJ45S | A keyed eight-position registered-jack configuration, as described by NECABLES. | It is not the same as the usual unkeyed modular connector used for Ethernet. |
| RJ48 | DigiKey describes it as an 8P8C connector used with shielded T1 data lines. | Shows that an eight-position, eight-contact connector can be used for a telecom interface other than ordinary Ethernet. |
The RJ48 example is a practical illustration, not a complete definition of every RJ48 variant. DigiKey’s connector overview associates RJ48 with shielded T1 data lines.
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Eight-pole connectors also appear in standards for telecommunications interfaces. The abstract for ISO/IEC 8877:1992 says it “Specifies the 8-pole connector (plug and jack) and the assignments of poles/contacts for use in physical interfaces.” ISO’s catalog says the edition was last reviewed and confirmed in 2002 and remains current. Its scope concerns ISDN basic access interfaces, not a blanket definition of every Ethernet plug.
A separate document, ISO/IEC TR 9578:1990, covers communication interface connectors used in local area networks. ISO reports that it was reviewed and confirmed in 2015; its broad scope does not establish a particular connector as the Ethernet standard.
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When to say RJ45 and when to say 8P8C
- In ordinary conversation: “RJ45 Ethernet port” or “RJ45 cable” is useful shorthand when the context makes clear you mean Ethernet.
- In a specification or parts list: Name the physical connector as 8P8C, then state the relevant cable and wiring standard. That communicates more than connector shape alone.
- When identifying an unfamiliar cable: Do not infer its application or pin assignment from an eight-position plug. Check its labeling, wiring, and the equipment it is intended to connect.
There is no universal rule established here that forbids “RJ45” in everyday speech. The objection is about precision: the Ethernet plug commonly given that name is generally an unkeyed 8P8C connector, while the registered-jack label refers to a particular interface configuration.
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