5 ms·
> The MAC address is meaningful only for a single link This isn't entirely true. In ye-olden days, Ethernet was a proper bus architecture, where you would have
by gizmo686 1y ago
> The MAC address is meaningful only for a single link
This isn't entirely true. In ye-olden days, Ethernet was a proper bus architecture, where you would have many devices talking on a single line.
As networks got larger, we wanted to have more devices than we could get on a single cable, so we introduced cheap booster hubs that would physically connect multiple cables. Since these hubs didn't have any fancy switching logic, it continued to look to devices like everyone was on a single bus (which they basically were).
Eventually, compute became cheaper, and the limitations of having a massive bus more apparent, so we started replacing some of the Ethernet hubs with Ethernet switches to reduce contention. It still looks as if everyone is on a single bus; but now a node only sees traffic involving a node on its side of the nearest switch. Notably, these switches were drop in replacements for hubs, so they did not rewrite MAC addresses.
Nowadays, actual Ethernet hubs are a dying breed (although I still keep one lying around to avoid buying an expensive managed switch with a monitor port). Almost every Ethernet cable you run into today is a genuine point-to-point connection connecting two specific nodes, and carrying only the data that needs to be sent between those nodes.
Now that modern Ethernet is a full featured packet switched protocol, it has to deal with all the routing issues that come with that. This led to the introduction of the Spanning Tree Protocol, to allow Ethernet switches to construct Ethernet routing tables. Notably, nothing inside of these Ethernet networks replaces MAC addresses. When computer A sends to computer B on the same Ethernet network, the destination MAC address A uses is B's actual MAC address, regardless of how many switches are between them.
- nemothekid 1y ago>so we started replacing some of the Ethernet hubs with Ethernet switches to reduce contention. I just remembered having to answer questions about Ethernet hubs vs switches many years ago. To me, ethernet hubs are entirely theoretical, I don't think I've ever seen one in the wild.
- kstrauser 1y agoYou weren’t missing much. They sucked. Switches were an improvement in every way.
- jedberg 1y agoThe really awesome part about Ethernet hubs was that if you knew what you were doing, you could get everything on the bus delivered to your device. Fun fact, in the 90s, most every college dorm had one hub per building (or if you were at MIT, one huge network for all of them). Since most everything ran over plain text, you'd be surprised what you could learn when you aggregated all AOL IMs and ICQ messages in one interface...
- esafak 1y agoGentlemen don't read each other's packets ;)
- jedberg 1y agoOf course not. But I was 18 and certainly not a gentleman. Teenagers are morons. :) I would never do such a thing now of course.
- dotancohen 1y agoNow that everything is https, I don't think that you could.
- jedberg 1y agoWhy do you think everything is https now? :)
- dotancohen 1y agoGo on, I'm interested...
- immibis 1y agoIt's because Firesheep made it easy to do exactly that
- immibis 1y agoSomeone recently did a YouTube demonstration of an Ethernet bridge: https://www.youtube.com/watch?v=Hvqv9QcTcfA https://www.youtube.com/watch?v=Hvqv9QcTcfA (skip to 15:22). It's a large, $8000 (in 1986), 10kg device with a 68000 processor that connects to exactly two ethernet busses via AUI ports. It even speaks an earlier variant of STP. It remembers a certain number of MAC addresses on each bus, and when it sees a packet on one bus addressed to a device on the other bus, or a broadcast, it stores it and sends it when the other bus is free. The idea of a two-port switch sounds silly until you realize there can be 30 devices on each port.
- Animats 1y agoOh, that thing. The next step up was the DEC DELNI, the same thing but with about 8 ports. Had one of those sitting under a raised floor. The main purpose of bridges like that was to overcome the electrical limitations of coax Ethernet. Cable losses, transceiver losses, and length limitations meant that beyond some length, you needed a power booster.