Wiring Ethernet through Telephone Cable

If it's a reasonably modern twisted pair telephone cable, then it's equivalent to CAT3 Ethernet. For Ethernet, that's obsolete by today's standards. It's only good for 10Mbps, or maybe 100Mbps over short runs.

By comparison, all the cables you can buy today are CAT5e (100Mbps to 1Gbps) or CAT6 (up to 10Gbps).

What this means is that your Ethernet connections may be slow by modern standards, or unreliable.

Older telephone cables that are not made up of twisted pairs are unlikely to work at all.

You can't do that. Modern ethernet requires point to point connections, not bus topology. And it is quite likely that since your telephone was wired bus topology that the wire quality is inadequate to even try using the phone cable for even a single computer connection.

Edit: Really for a 100% accurate answer to your options we need to know What kind of phone cable (type and number of wires) and what is your internet source. There are some extremely rare situations where some answers might miss some narrow options. Like if your source is DSL, you could make internet available at any jack, but you could only connect one jack at a time to a computer. Or if internet from a coax you may be able to place a router at one location that has multiple taps and make ethernet available at multiple locations, but not through any downstream taps.

Nope! Plain Old Telephone Service (POTS) jacks were usually daisy chained as you describe. Ethernet requires a "home run" from each jack to a modem, router or switch. Also, it's probably cat-3 which is unsuitable for Ethernet.

EDIT: Based on comments (which sometimes go away, but answers don't), I'm adding to my answer here: According to others Cat-3 can be used for limited ethernet service (I wouldn't recommend it), but every jack would have to be wired in a home run configuration, which is uncommon for POTS wiring. Next: As others commented, the POTS wiring might not even be Cat-3, it could be 4 wire cable (Red, Green, Yellow, Black) which was very commonly used back in the day, which wouldn't work at all.

There is a separate issue of the type of cable. Telephone can run on CAT 3 (10 Meg. ethernet), CAT 5 (100 Meg. ethernet), CAT 5e (1 Gig. ethernet) but also on much older types of cables that are not suitable for ethernet at all. Assuming that the cable you already have in place is at least Cat 5 (100 Meg., which is arguably enough for typical internet usage), the topology is the next challenge.

Standard analog telephone service (POTS) could have many devices - telephones, answering machines, fax machines, modems, etc. - on a single line. All devices are connected together with a pair of wires. The polarity of the wires usually doesn't matter, and the devices could be wired in a star topology (a bunch of cables joined together at one end with a device on each cable on the other end) or chained one device (or more commonly, wall jack for a device to plug in) to the next. A single chain through a building is generally cheaper to install (less wire, a lot less labor if installed while the walls are open during construction) than a star configuration, so until home networks became common, that was the standard installation method. (Exception: the house I grew up in actually had separate runs to each room, though that was in the days before twisted pair ethernet, so it didn't matter much).

Originally, ethernet used coaxial cables with a tap into the cable for each computer. This was replaced many, many years ago, by twisted pair cables using hubs and switches. With twisted pair cabling (as used for ethernet, there are other varieties for other systems that are not applicable here) with each cable providing a single point-to-point connection, generally between a computer and a hub or a switch. (Almost all ethernet networks now use switches, not hubs, but that doesn't actually matter much for this discussion.) There are also point-to-point connections between switches, between switches and routers, etc. A typical magic internet box WiFi router is actually 3 devices in one: Router, WiFi access point and ethernet switch.

The end result is that daisy-chain telephone wiring is extremely common but also nearly useless for ethernet networking. The exceptions are:

• If the cables are high-enough quality (at least Cat 5) then you can often get one good connection per chain.
• Even if the cables are below modern ethernet quality, you may be able to use some sort of network extender to get a good signal to one location.
• If the cables are loose then you may be able to use the old cables to help pull new cables.

If I can weigh in here. I have only used cat5 and up in my professional life, but we did do some hacks in a pinch.

If you give more info (why you do it this way, photos of cables etc) I could be more help.

If the cabling was done in a star configuration (all connected to a central point) then you are at least lucky in that regard.

Firstly lets start with the cable type. Ethernet cable is twisted in a way that minimizes electromagnetic interference. Depending on the type of cable installed in your place you may get away with using 4 wires per connection. This would mean connecting your RJ45 connectors pin 1,2,3 and 6 matching on each side .If you have 8 wires (4 pairs) then just match both sides. You can also make a direct connection between 2 network nodes (pc's, pc to printer etc) by connecting 1 to 3, 2 to 6, 3 to 1, 2 to 6. This is called a cross over cable. I once had to run 2 seperate connections over 1 cat5e this way just to get a temp net up and running(don't ask).

Sadly this will give you at best a pathetic 10mbps. To make this into an actual (terrible) network you would need to install a network switch at the central point and connect your newly crimped connectors.

Best option if you plan to remove/repurpose the telephone wiring in your place (check if legal!) , just buy(or salvage / ask at sites where buildings are decommissioned or wiring replace ) some cat5e or cat6(better) and use the old telephone wires to pull the new cables through. Use proper connectors and a switch( ask around if no budget) and you will be good to go. Your future self will thank you.

Still if you are just doing this to see if it works, great! Messing with stuff is how we learn.

Can you run Ethernet over an old phone cable? Not officially, but possibly in practice.

You can run 10BASE-T and 100BASE-TX Ethernet over any two wire pairs (the other two wire pairs in a standard "RJ45"/8P8C Ethernet connector are unused until you get to Gigabit Ethernet).

The standard requires twisted pair cables (to reduce interference), which most older phone cables are not, but for short runs (say, less than 20 meters), you may be able to get away with it. It depends on both the cable quality and endpoint hardware.

Can you connect three or more Ethernet devices to the same physical wire (daisy-chain/bus)? With ancient (1980s) coax Ethernet, yes, but not with modern "RJ45" form-factor Ethernet. Connecting just two devices and leaving the other phone sockets unconnected might work, though the unused sockets do add interference.

TL;DR: It may be feasible to connect two devices, but you won't know before you try.

If you want wired Ethernet without dragging new cables, you're probably better off looking at an Ethernet-over-powerline solution like HomePlug.