RJ45 to RJ11 Converter Adapter Connector M/F Cable,Uvital Telephone RJ11 6P4C Female to Ethernet RJ45 8P8C Male Converter Cord(2 Pack)

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For 10/100 Ethernet to work, you need to have a straight through connection of pins 1, 2, 3, and 6, and the other pins are not relevant/not used in this scenario. The answer is possibly... but without knowing more about the cable and the layout it is difficult to tell.

Rj11 Adaptors | CPC UK

In the 568B color code, we need W/O-O/W (Pins 1&2) and W/G-G/W pairs (Pins 3&6) Pins 4, 5, 7, and 8 can be ignored (they are for PoE and Gigabit applications, which we are not going attempt with this type of cable). Enabling equipment with a RJ45 type plug to be used on RJ11 wiring systems that are now commonly used in offices and public buildings for Broadband . The most commonly used is the four-wire version listed hereDisplaying products for Rj11 Adaptors - from our range of leading brand manufacturers. Rj11 RJ45 Adaptors

RJ11 to RJ45 - Super User networking - Convert RJ11 to RJ45 - Super User

In theory, telephony cable can be used to support 10/100 connections (not Gigabit) over short distances. If you can find a telephony cable that is not cut and is a "home run" between the two points you need to connect, and it contains at least 4 conductors, it may be possible.