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PaperclipPerfector  3d ago (text post)   2248 thread views

Small-Scale Question Sunday for January 5, 2025

Do you have a dumb question that you're kind of embarrassed to ask in the main thread? Is there something you're just not sure about?

This is your opportunity to ask questions. No question too simple or too silly.

Culture war topics are accepted, and proposals for a better intro post are appreciated.

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Default gateway does what it literally says. A gateway, in IP routing, is a term meaning "traffic for X network should be sent to the router at Y destination IP address". You can have potentially many routes on a system specifying what traffic goes where. The default gateway, then, is the gateway which your traffic will use when no other routing rule matches.

I'm not certain about what happens when two machines claim to have the same IP, actually. But I can take an educated guess. When you try to reach out to an IP address, your machine first needs to figure out which Ethernet MAC address it should send that traffic to (it does this using a protocol called ARP). Most likely, what would happen is you would start to see traffic for that one IP address go to both machines sporadically, depending on which is responding to ARP requests first. I'm not certain but that's what I would imagine.

DHCP works by sending out a broadcast on Ethernet asking for a DHCP server. When the server replies, it will give the client an IP address to use. That's the gist, though I don't know the exact details of the DHCP communication (I couldn't write my own software or anything).

UDP goes through NAT the same as TCP does. If you're making an outbound connection, the router will pick a port to listen for reply traffic, and forward replies to your client machine. If you're making an inbound connection, you need a port forwarded to the destination at the router level in advance.

A VLAN is a way to isolate Ethernet networks even if they are plugged into the same physical hardware. The switch you are plugged into lets you configure which ports are part of which VLAN, and only ports which are part of the same VLAN can talk to each other using Ethernet. You can also configure a port so that multiple VLANs are allowed, in which case the device plugged into the port must add a tag to any traffic it sends specifying which VLAN it is for (and it is only allowed to send traffic on the VLANs you configured on the switch).

I'm not certain about what happens when two machines claim to have the same IP, actually

Depends on the IP. If it's so called "local" IP (starts with 10., 192.168 or 172.16.) and they are not on the same local network, then nothing bad, since these addresses specifically designed for such use. If they are on the same local network, there would be trouble, not sure about the exact nature but likely both computers sharing an IP won't be able to properly use it. Usually your OS would scream at you in some way when such thing is detected. Using DHCP server is one of the ways to ensure this thing never happens.

If you have two hosts that have same IP and those are not local IPs then weird things would happen. In general, if you have NAT (which most home users for now should and would have) then outgoing connections should work fine (then again, there's no real reason for a machine used by home user to even have a non-local IP at all) but it's better to avoid that situation completely because things get weird. There are special organizations and protocols aimed at segregating IP space so nobody steps on each other's toes there. As a home user, you probably don't need any of that as the standard setup is to use local IPs for everything inside the home network and only use non-local IP for the main router egress address.

Yeah, I meant two machines on the local network. I've never tried that one before. I unfortunately have first-hand experience with machines on different networks using the same IP addresses, as one of my old employers was too cheap to buy new IPv4 subnets and we squatted on the DoD 22.0.0.0/8 subnet. Our network team had a very fun day when the DoD started using that subnet publicly, or so I heard from my old coworkers (I had left by that time).

DHCP server restarts can cause IP conflicts pretty often, especially if you're running the DHCP server on a small home/office router that doesn't persist state. Windows will specifically warn about the IP conflict, and newer (Win7+) will often try to automatically reregister with your DHCP server if you're not running in static modes; Linux has some optional standards-complaint IP conflict notifiers.

If not corrected, the usual results are inconsistent communication and higher network utilization: network switches will resolve the IP address to physical multiple ports, and this causes packets to be sent many more places than they need, and can sometimes cause TCP connections to go wonky.

((There are exceptions and sometimes even cases where you could use this behavior, but they were always rare and increasingly have been replaced by better solutions.))

DHCP server restarts can cause IP conflicts pretty often, especially if you're running the DHCP server on a small home/office router that doesn't persist state

More fun can be had if there's a rogue DHCP server on the network. Back in the days I did network admining work (a long time ago) I had to deal with such a case - turned out to be a new printer with helpful on-by-default DHCP server, but it took me a lot of frustration to figure it out because I never thought before a printer could do that to me.

What idiot thought it was a good idea to add a DHCP server to a printer? That is peak anti-social.

In an office setting, I know it would take me so much time to try to figure that out. At least in a home setting, it's far easier to remember the answer to "what was the last thing that I (or spouse or kids) attached to the network in the past day or two?"

Yeah that was my question exactly when I finally discovered what happened - who even thought it was a good idea to do this? Thankfully, haven't heard about someone doing that for a long while now.

What idiot thought it was a good idea to add a DHCP server to a printer?

It's probably so you can connect directly to the printer, without needing a router. Of course, it might be smarter to first try to acquire a DHCP lease before starting a DHCP server...

Yeah I think it was some kind of "smart" home solution when not everybody had routers on home network. A bit fuzzy on details now but that might be the idea at least. It had a normal "play nice" mode too, just for some reason it wasn't enabled by default... or maybe somebody switched it for some reason, impossible to know now.

Oh yeah that's a good time too. When I was in college, I worked in the IT department and every so often we had to deal with a student who brought in a router and plugged one of their LAN ports into the campus network. That got your port shut off by the school pretty quickly, and iirc you had to come in and promise to stop using your router to get it turned back on.

Students are something else. I am still ashamed of some of the shenanigans I did as a student, especially after eventually finding myself on the other side (not in an university, thankfully). It's a tough job to run IT in such places.

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