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  • Hacker News
  • > There is one DHCP server.

    There'd better be or your network is probably hosed.

    by ronsor
  • You don’t have to have one. My first LAN didn’t. I had no idea what I was doing and picked 17.0.0.0 addresses for local ones and I couldn’t connect to Apples website.
    by briandw
  • Well, consumer Windowses (XP Home? It’s been a while) used to hose networks because they were "helpfully" running their own DHCP server.
    by Sharlin
  • That works mostly, you'll have problems if you have duplicate IP addresses though.
    by Hikikomori
  • Windows supports dhcp failover, where multiple dhcp servers can exist on the same subnet.
    by kryogen1c
  • What really matters is that there's a coherent policy for assigning addresses, which is absolutely compatible with having multiple DHCP servers on the same subnet (e.g. each server configured to hand out non-overlapping IP ranges).
    by 0xcde4c3db
  • https://datatracker.ietf.org/doc/html/draft-ietf-dhc-failove...
    by smkelly
  • At work I manage a network with two DHCP servers. One is a dnsmasq that assigns the IP address, hostname, and basic network parameters to all network devices, and the other is a LTSP server (https://ltsp.org/) that provides network boot images only for certain devices that require it. It might not look pretty, but it works really well.
    by pezezin
  • In theory it's fine to have multiple DHCP servers as long as they don't step on each other. But in reality a bunch of the things mentioned in this post are ways for the network to be hosed. Unfortunately, enough LANs are semi-hosed that it's worth working around known problems.
    by wmf
  • This just lists every possible configuration permutation. Nothing insightful here.
    by suprjami
  • >Two hosts on the same subnet can reach each other without leaving the LAN.

    Barring "very buggy software," when is this false?

    by StilesCrisis
  • Yeah… I feel like “falsehoods X believe about Y” need to omit anything a reasonable X would consider to be a bug.

    Edit: fair point. Maybe not.

    by Waterluvian
  • I feel like the better of this is maybe "will" instead of "can"
    by justinlivi
  • Some tunnel that extends the subnet somewhere else. Guess it depends on how you define lan.
    by Hikikomori
  • LAN spread across sites with a WAN in the middle of it. But that's why I said in my comment that a LAN isn't actually a real thing.
    by mitxela
  • Most consumer wifi routers I’ve used have a setting to prevent peer-to-peer traffic. Whether this still counts as being on the same subnet is kind of a philosophical question.
    by taneq
  • Switches and firewalls can do all kinds of things. Probably the most common case is that public/guest wifi networks often have client isolation set up so that different devices on the network can't contact each other. Same subnet, but only access to the gateway and the internet in practice.

    (Also can happen if ARP is not working correctly, which can be a right PITA to debug).

    by rcxdude
  • There are recent "client isolation" features that prevent P2P communication. Some hosting providers also block customers on the same LAN from attacking each other.
    by wmf
  • Port- or Client-isolation on Ethernet switches and Wi-Fi access points respectively. Often seen in corporate environments and (properly configured) guest networks.
    by dipierro
  • Some of these seem a bit obscure. I wish the author had given detailed examples or linked to pages with details on the points.
    by AnonC
  • I love the number of people who are commenting about how the article is wrong, and then finding out that they are actually wrong.
    by jyounker
  • > My LAN uses IP addresses.

    I've never heard of a LAN without IP addresses. Is that a thing? I'd assume that no IP addresses means you can't even use TCP/IP at all, right? Is there something else you'd use instead?

    by LoganDark
  • IPX/SPX is not IP based.
    by solraph
  • Quite common to run unnumbered IS-IS backbones. Whether this can rightfully be considered a "LAN" is up for debate.
    by peanut-walrus
  • You can send packets over Ethernet, without IP. That's what people did before IP was invented and took over most networked communication. It's less convenient, because you can't reuse the IP stack, so almost nobody does.
    by mitxela
  • I'm not sure how much of a "LAN" that would be but some telecom clustering solutions use protocols other than TCP/IP (some don't even care if they run over Ethernet or ATM)
    by yaris
  • Not sure what a "LAN" is but it happened to me at work that I thought .0 is not a valid host address until I learned about Layer-3 only networks. So also no ARP. I guess this is still a LAN although it's probably an "Enterprise LAN".
    by ofrzeta
  • > Layer-3 only

    huh? 1.2.3.0/23 is a valid host address, it doesn't have to be Layer-3 only or whatever. it's just basic subnetting.

    by allarm
  • * there is something called a LAN

    * two people who say they are on the same LAN agree on what the LAN is

    by mitxela
  • This feels like a bastardization of the already questionable "Falsehoods ___ believe about ___" format.
    by elaida73