hey so most home routers are just. a DHCP client. right? like, the service i pay for is technically that my ISP will run something in my home and install fixtures (e.g. a modem) that eventually give me a singular Ethernet cable. some(most?) ISPs will also provide a router; but i can use my own.

but ethernet can easily connect to multiple devices, right? on the other end of that router, usually i have a switch going into multiple devices, all being DHCP clients of that router. it’d be silly to have just one port (as my router does), plug in one device, and say “welp that’s the network exhausted”. of course you can connect many clients.

and my ISP is fundamentally also just a DHCP server over ethernet, right?

if i plug it my modem into a switch, and connect, say, TWO routers, and they’re both DHCP clients, what do ISPs usually do? surely that doesn’t work? assuming no CGNAT, i’m not just gonna get two assignments, and therefore two public IP addresses, right??

@sodiboo it's per-port on their end, plus might also be using PPPoE

@sodiboo correct, they'll only respond to one device

@freya ooh okay i’ve heard the term PPPoE before but didn’t know what it was. “Point-to-Point Protocol over Ethernet”. From what i can tell, it’s kinda sorta vaguely like DHCP, except it’s “point-to-point” (single client only), and it requires authentication and provides encryption (both to prove that i am indeed a customer, and to protect my physical connection from the malicious actor down the street; all my data is encrypted to the ISP’s datacenter, that’s really cool!)

So i guess this means that my modem is actually the DHCP server, right? It’s not just “turning the raw fiber into Ethernet”. Turns out, that little box is actually doing something, who would’a thunk it!

And i guess that means it’s likely running a DHCP server which only really expects a single client: The modem is assigned at most one IP address from my ISP, and generally just delegates all connections to my router whenever the router is connected. So, whatever happens when i connect multiple routers is probably “it depends on the exact modem how exactly it freaks out, but the ISP doesn’t even know that this happened and still just sees a single PPPoE client”.

Is that about right?

@sodiboo when we got a custom router we had to clone the MAC address from our ISP provided router in order to get it to work, I assume this is standard practice for them to check the MAC address of the router they’re talking to in someone’s home. so either it will only talk to the router with the correct MAC address or you’ll have MAC address collision and I have no idea what happens then, but most likely not two IP assignments

@cuddlebug I know my ISP definitely doesn’t care about the MAC address of the router because like… i simply do not have their router. don’t even know where it is. never did any cloning of anything.

But something that might be worth mentioning though, is that in basically all literature about home networking, it’s that “The ISP runs fiber to your home”. In my case, this literally isn’t true? In some parts of Västra Götaland, we have a physical network maintainer called Streemi. They’re an exchange between customers (homes) and ISPs (also broadband providers; generally all “data” services, not just internet). There is no “default” ISP here, and it’s easy to switch. It’s literally just a phone call and changing a payment plan. Sometimes they come with a router or a streaming box; sometimes they don’t. We have like three different streaming boxes from three different providers; and we also have at least two routers we got that we never ever plugged in because we already had one.

I’m not sure how exactly this works behind the scenes, but i assume that this openness comes with some subtle benefits, like the fact that i’ve never really thought about situations like “The connection might be locked to your router’s MAC address, and no other devices are accepted. If you connect two of the same device, obviously ethernet just dies”. Like, that didn’t occur to me as a possibility, and it genuinely feels so foreign to me. Mainly because these kinds of restrictions reek of the “i live in a Comcast neighborhood” kinda vibes i’ve heard is normal in some countries; where you have a single option for an ISP and if you wanna switch then Fuck You. You Can’t.

So, maybe the true answer to my question of “What happens if you plug in two routers?” really is just “Try it and see!” because wow. In your case the answer is essentially “you literally can’t connect two routers”, but in my case i know i can surely connect them; i just don’t know how the modem and/or upstream provider will react to such a scenario.

@sodiboo @freya PPP assigns the IP as part of the negotiation, and generally won't allow / comprehend multiple clients using the same credentials

@sodiboo @freya tangentially I'm using the ISP provided VDSL router in bridge mode to convert PPPoE (protocol) over VDSL to PPPoE (actual ethernet) which is then ingested and VLAN'd by my switch and passed to my router on one of the 10G VLAN trunk connections

@izzy @freya Yeah, but also, i only have one PPP link in my home? The fiber cable that goes into the modem. I don’t even know how to connect multiple things to that. But yeah, i guess if the link to the ISP only understands the concept of a single client, then the answer of how exactly the box in my home breaks, is probably not that interesting.

@sodiboo they should do an isp for pros where they just give you a working internet hole

@tauon

i think “isp for pros” is called Arelion

@sodiboo @freya well how it does it is the same as any other instance of NAT
the only weirdness is that your internet link happens to be encapsulated in PPP

@izzy @freya But my router gets a public IP address, and all ports are automatically forwarded to it. I don’t think it is behind NAT. My modem almost certainly won’t just do NAT and allow both clients unrestricted internet connectivity. Because then, how would it know which one an incoming connection is routed to?

@sodiboo what’s that

@sodiboo either it’s exhausted after 1 device, or it’s locked to a specific mac address (specifically the one of the first device you connected)

@tauon a swedish “tier 1 network provider”, part of the internet backbone infrastructure. they are the (i believe only) upstream network of Telia. so just as my home is only connected to Telia (Telia is my ISP), Telia is only connected to Arelion (Arelion is their ISP).

it’s, quite literally, an “ISP for pros”.

a “tier 1 network provider” in this context means that there is no upstream provider; “internet connectivity” means “being able to send packets to a tier 1 provider”. they are the internet.

@sodiboo @freya the ISP is not forwarding ports to your router, they are routing packets to it
your router would be doing the NAT assuming a normal setup of one IP for your connection and a private local IPv4 network

and generally said router will allow unrestricted outbound but have no port forwardings configured through the NAT

@sodiboo @freya once you strip the PPP you can treat the DSL/fibre connection as just an abstract IP connection, not unlike a simple usecase for ethernet

@sodiboo so the answer is a bit more complicated than one might think

assuming no cgnat, it might actually just work and assign two different public (in the sense of non RFC1918) addresses

old Bredbandsbolaget buildings, serviced by Telenor does this for a really silly reason, their IPTV boxes don’t work with the routers they ship, because they use IGMP to “subscribe” to multicast groups for specific channels/streams, and their routers don’t come with an IGMP proxy (to properly handle the igmp and multicast traffic through nat) so they just give you two addresses, one for the router and one for the tv box directly, but they’re not Mac address locked, nor are the two addresses functionally different

another example is Telia, since they were early internet adopters they have (or at least used to have) a TON of ipv4 addresses, so they just gave the parents residential area a /29 per house

now that being said, cgnat is becoming more and more widespread (especially here) because ipv6 rollout is painfully slow (thank PTS for that one >:c /s)

@izzy @freya

the ISP is not forwarding ports to your router, they are routing packets to it

well, no, that is not the exact topology: there is a modem inbetween. the modem is (presumably?) using PPPoE, and out the other end pops DHCP, which expects a single router. that router gets packets unrestricted, without any NAT in front of it.

your router would be doing the NAT

yeah, but the question here is “what if i have two routers”. my modem provides an ethernet link to my router with DHCP. DHCP supports multiple clients.

the hypothesis is that my modem is essentially assigned a single IP address, and there is no way around this. i therefore deliberately say “all ports are forwarded to my router”, because it makes more sense to me that the IP address is “owned” by the modem. the ISP routes packets to my home, yes, but after that, it goes through a DHCP server that… doesn’t do NAT i guess? but it feels like it could do NAT? but it clearly doesn’t, because if it did do NAT; then it would also implicitly forward all ports to its one and only client.

if i connect another router next to it. doing NAT on two clients is definitely not correct in any way, because then my router will not unconditionally receive all incoming packets. the modem would have no way to know which one to route a given packet to: i.e. which port is forwarded to which router in my home.

once you strip the PPP you can treat the DSL/fibre connection as just an abstract IP connection

Yeah. But my router isn’t behind an abstract IP connection in this sense. It is connected to a DHCP server.

I realize now, that, if the modem is the DHCP server, and the modem does only hold a single abstract IP connection, then the answer is perhaps a boring one that “the box in my home just breaks in a weird way”.

@thermia Okay, actually, screw connecting two routers to the “internet” port: the idea of the tv box getting a public IP address sounds fucking hilarious. Yes. My modem literally does have two ethernet ports; one for the set-top box, and one for the internet router.

If my home actually gets exactly two public IP addresses, and it’s for this reason, then that would be incredibly fun and exploitable!

I’m also curious, is my region just weird? You’re swedish. What you describe here sounds familiar. But you live in like, stockholm or something. Here, we have a state-owned fiber network for last-mile internet delivery. Is a similar system used where you live? Does what i describe sound familiar, fellow swede?

@sodiboo so here’s the answer, it doesn’t hurt to try! ^-^

@izzy @sodiboo @freya yeah PPP hasn’t been used here since after the DSL times :3

@thermia @sodiboo @freya it's always DSL times in Australia

@izzy @sodiboo @freya ough, my condolences

@thermia @sodiboo @freya "australians don't want faster internet" ~ scott "scummo" morrison, PM of australia, 2015 or so

@izzy @sodiboo @freya oh my god :(

@izzy @sodiboo @thermia @freya yea obviously burning coal is much more important /s

@sodiboo @freya ohhh that's a really weird setup my bad

I figure there's three possible ways

- some sort of transparent-ish PPP connection that the modem-router strips to present DHCP from the ISP side (with a range of 1)
- the modem-router is doing DHCP and handing out public addresses. which is weird but not impossible, and potentially wasteful and/or not spec compliant if they're re-using the same address for multiple modems?
- the modem-router is doing a single-address DHCP server on a private subnet then forwarding all packets to that address. I guess.

these are all weird and I want to say more cursed than just using a modem to bridge PPPoE over VDSL to your ethernet network

but I don't really have a good understanding and could be wrong. this also all assumes PPPoE. I don't really know how stuff behaves with IPoE style fibre connections I'm yet to interact with one

@sodiboo yeah this is quite common practice! one company owns the ftth line, but they’re not necessarily an ISP themselves. this is commonly referred to as a “stadsnät”, ISPs in turn then connect themselves to the city’s network (see: openuniverse), giving a great selection of ISPs, all for similar pricing in all areas!

it’s not as common in big cities because usually an area is serviced by whichever company got to the area first

parents live in an area where Telia owns the physical fiber, but Telia allows other companies to lease the line, so one can choose another provider BUT they make sure the leasing price is more expensive for everyone else, meaning Telia can offer wayyy cheaper plans

@thermia crying at the thought of Telia-owned anticompetitive stadsnät

@sodiboo they literally call it “Telia Öppen Fiber”

@tauon @sodiboo internet hole is my favourite kind of hole

@sodiboo depends on the ISP! mine sets a limit of 5 public IPs/DHCP clients per account for example

@taavi that’s a fascinating limit! how did you find out that it was 5? do they just document that? or did you experiment?

@sodiboo documented somewhere iirc, although I can't find that anymore. I've had uses for 2 or 3 in the past, but never the full 5