Long Reach Networking
This issue isn't necessarily specific to 68k Macintoshes. There is not yet any information about whether or not AppleTalk will flow over these devices. AFP over TCP/IP on systems that have OpenTransport should have no problems, of course. I haven't looked into what's available for extending the reach of AppleTalk over LocalTalk/PhoneNet wiring.
Also, this particular page, and my research so far, is exclusively about wired technologies that bridge Ethernet to DSL. Even longer distances will be possible with fiber, and of course ethernet on its own is typically good for several hundred feet.
My research into DSL technologies has recently led me to discover that both SHDSL.bis and the VDSL2 standards have been used somewhat extensively in easy to set up, point to point ethernet extension technologies.
There are a few companies that make each of these things, but the most reputable is probably ZyXEL.
The first notable device is the P871M, which supports up to about 100 megabits at fairly close range, and is advertised as having a distance range of about 1.5KM. It should be good for bringing most or all of the speed of your Internet connection to the shack in the back or a distant corner of a property. If you have a very close neighbor, you may also be able to connect their house to your LAN using two of these devices. They sell for shy of $220 on Newegg.
The main question with VDSL 2 is that it is a widely deployed standard. However, there are eight different "profiles." Essentially, how VDSL2 works is the same as how most DSL technologies work -- they send signals at a frequency that's probably pretty difficult for humans to hear, but VDSL2 uses a whole bunch more bandwidth on the line, a larger amount of the audio frequency spectrum. The P871 supports profiles 8a and 12a.
Two $220 devices, plus possibly switches on both ends is expensive. It may be possible to use a P871M in "CO" mode on one end of the VDSL2 loop and something less expensive like a CenturyLink VDSL2 device on the client of the loop. The C1000a is available in many Buys, and online. My personal theory is that if you switch the device from PPPoE to DHCP mode and then put it into bridged mode, it should look and feel like an extension of the LAN in your home, up to 1.5KM away. ZyXEL has few of their own unbranded devices with switches and wifi, but the C1000a is going to be one of the easier solutions, if for no other reason than it is easy to buy. (If you are in a Verizon FiOS/DSL area, or an AT&T DSL area, I don't know if Best Buy will stock this modem. Verizon doesn't use VDSL2 anymore, if they ever did, and AT&T does not sell their VDSL2 devices at retail.)
VDSL2 has several profiles. Most equipment you can buy inexpensively from ZyXEL or Planet will support profiles 17A and 30A, but many ISP deployments are using an 8 or 12 profile, at least for long distance.
CenturyLink in particular will serve most of its customers on 8A profiles, though higher profiles may be used either for customers far away from the DSLAM (8B) or very closer to the DSLAM which would like a higher speed (17A/30A) and require less transmit power.
When you connect two ZyXEL P871Ms together, set one of them to "CO" mode and the other to "CPE" mode. They will automatically negotiate a transmission mode and a sync rate.
The ActionTec (CenturyLink) C1000A supports VDSL2 line modes 8A, 8B, 12A, 12B, 17A(U0) and 30A and should therefore be compatible as the client to a device such as the ZyXEL P871M. Various settings will let you control whether you want a remote C1000a (or other similar device such as the Q1000) to be a sub-network with its own NAT, DHCP, etc, or a transparent bridge. In addition, although the wireless on the C1000A should be reasonable, it can be disabled. It has four gigabit ethernet ports.
The ZyXEL P871M seems to be almost a pure bridge, at least by default. If you have a DB9 serial cable, you can configure them, but they should work out of the box
But what if you need more distance, or are interested in having two client locations?
SHDSL.bis is a pretty old standard. Officially, you can achieve about 5.6 megabits under optimal conditions, but the speed is completely synchronous. Compare to ADSL's official top speed of about 8/1M, ADSL2+'s top speed of 24/1, and VDSL2 17A's top speed of about 100 by 50 megabits. In addition, SHDSL has very favorable distance properties. While ZyXEL suggests that its VDSL2 devices can communicate at up to about 1.5KM, it doesn't list a max distance for SHDSL. Another site suggsts that using high quality copper, you can run SHDSL up to about 16 kilometers. If you and a friend live on the same road, almost ten miles from one another, you can have a direct, wired, 192 kilobit per second link to their house. If another friend lives ten miles in the other way, the P793H has some cool functionality. (Some other sites suggest it's about 8KM, I'm not sure if that was counting bonding or if there are different versions of SHDSL.)
The P793H is a device of many purposes. Firstly, if you live near an ISP with SHDSL service, you can buy a line from them and connect this device as a modem and gateway. Fairly straightforward. If your DSLAM has enough capacity, you can buy bonded service and connect the P793H to that service for double the throughput. (Just over 11 megabits in total.)
You can also connect two P793Hs directly together in one of three ways: 1) Two P793Hs linked by one pair of wires. You'll get 192k to 5.6 megabits-ish of throughput. 2) Two P793Hs linked by two pairs of wires. You'll get about double the throughput. 3) One P793H linked acting as a server two two other P793Hs,
There's some configuration involved on a web interface, but this is all documented in the manual, which is located on ZyXEL's FTP site here: ftp://ftp.zyxel.com/P-793H_v2/
The P793H explicitly states compatibility with other SHDSL.bis devices. It's worth noting that plain SHDSL seems to have a maximum data rate of about 2.3 Megabits, and SHDSL.bis will go up to about 5.3, all on a single wire. In addition, the P793H specifically is recommended for its ability to host the sessions of two SHDSL.bis clients.