Every type of cable has a maximum distance. These distance limits can vary greatly from one type of cable to the next. Along with determining whether a cable will work, distance limits will also determine how well a cable works. Knowing the fundamentals behind cable distance limits is the first step in selecting the best cable for your needs.

For Ethernet copper cabling when used for 10/100/1000BASE-T, the maximum allowed length of a Cat 5e / Cat 6 cable is 100 meters (328 ft). This consists of 90 meters (295 ft) of solid “horizontal” cabling between the patch panel and the wall jack, plus 10 meters (33 ft) of stranded patch cable between each jack and the attached device.

It must be 100m (328ft) or less in length to be certified. It is possible a longer cable will work, but it is not guaranteed. Shorter Cat 5 cables may also not work if there is a lot of Electromagnetic interference (EMI). Signal attenuation appears to be the limiting factor, too much signal loss and you can’t guarantee 100 megabits per second.

1-CH Passive IP Extender over Coax (IPVE702)
https://www.youtube.com/watch?v=FawhgMf3-Dw

Coaxial IP EXTENDER is designed to extend IP camera signal over existing coaxial cable, used in pairs with RG59 cable to exceed the IEEE 802.3 10Base-T NEX specification. Transmission distance reaches max up to 220m meters at 10/100 Mbps speed.
Coaxial IP EXTENDER is low cost and easy, simple and speedy migration from Analog to IP camera’s using existing coax cable.

Hmm… I thought that I posted an answer earlier but maybe I didn’t. Anyhoo, CAT3 cable will support 10mbps Ethernet (10BASE-T) but not FastEthernet (100BASE-TX) so it should be possible to use it for Ethernet if there are enough pairs and if the connectors are terminated correctly. It doesn’t matter which wires you use as long as you use the same colors on both sides of the “run” and terminate the connectors correctly.

As someone who is doing this currently, I can say that it does work. As Scott pointed out, you only actually use 2 of the pairs for standard ethernet.

You will obviously get better performance from Cat5, but your landlord might have something to say about ripping out the current wiring. Over a short distance, you might even get 100Mb out of it.

Other answers here explore the possibility to use the Cat3 phone cabling for 10 Mbit/s (10BASE-T) Ethernet only.

There are ways to use Cat3 phone cables at higher speeds, as listed below. They do not run Ethernet over the cable but protocols more adapted to the phone cable medium, but all provide Ethernet at the connector sockets used to access these connections. So for practical purposes, it does not matter that it’s not Ethernet.
(1) G.hn over twisted pair

G.hn is a standard for home networking over legacy cabling (phone cables, coax and power lines). So, exactly made for the purpose at hand. It provides up to 2 Gbit/s, with commercial devices for phone lines typically providing 1 Gbit/s. That’s the PHY layer signalling rate though, while the usable data rate on the IP layer would be 400 Mbit/s (source). The transmission distance is up to 1000 m.

One nice attribute of G.hn is that it’s not limited to point-to-point connections like VDSL modems are.

(2) VDSL P2P modems

(3) HomePNA

HomePNA was a competing standard to G.hn, but their standards group merged in 2013 with the HomeGrid forum behind their G.hn standard (source). So HomePNA is considered legacy now.

Tut Systems sells baluns that can be used to use phone wire for low-speed ethernet, however, using wifi is going to give better performance. You could use VDSL adaptors, but, the cost of those for the short range 100mb/sec is going to exceed the cost of getting 802.11n adaptors and a gateway.

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Yes, you can have 10Mbit/s etherhet and for that you only need 2 pares (4wires).

I have actually done this in the past, and 10baseT was design to work on phone lines.

But if you would like higher speeds…. newer cables.

Plain old telephone cable (two twisted pairs) is good for 10Mbit Ethernet for runs of 10..20 metres. Your switch will automaticall fall down to 10Mbit if the quality of the transmission is not suitable for higher speeds.