this post was submitted on 07 Apr 2025
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Basic networking/subnetting question. (lemmy.dbzer0.com)
submitted 3 weeks ago by marauding_gibberish142@lemmy.dbzer0.com to c/selfhosted@lemmy.world
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Sorry for being such a noob. My networking is not very strong, thought I'd ask the fine folks here.

Let's say I have a Linux box working as a router and a dumb switch (I.e. L2 only). I have 2 PCs that I would like to keep separated and not let them talk to each other.

Can I plug these two PCs into the switch, configure their interfaces with IPs from different subnets, and configure the relevant sub-interfaces and ACLs (to prevent inter-subnet communication through the router) on the Linux router?

What I'm asking is; do I really need VLANs? I do need to segregate networks but I do not trust the operating systems running on these switches which can do L3 routing.

If you have a better solution than what I described which can scale with the number of computers, please let me know. Unfortunately, networking below L3 is still fuzzy in my head.

Thanks!

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[–] neidu3@sh.itjust.works 3 points 3 weeks ago* (last edited 3 weeks ago) (1 children)

Yes, but that's done on the switch. Basically VLAN tags are applied in one of two ways:

Untagged (sometimes called Access) is something you apply on a switch port. For example, if you assign a port to Untagged VLAN 32, anything connected to that port will only be able to see traffic assigned to VLAN 32.

Tagged (sometimesreferred to as Trunk), on the other hand, is for traffic that is already assigned a VLAN tag. For example Tagged 32 means that it will allow traffic that already has a VLAN tag of 32. It is possible to assign multiple VLANs to a Tagged port. Whatever is connected to that port will need to be able to talk to the associated VLAN(s).

In your particular case, the best practice would be to assign two ports (One for each host, obviously) to Untagged 32 (arbitrarily chosen number, any VLAN ID will do, as long as you're consistent), and all the other ports as Untagged to a different VLAN ID. That way the switch will effectively contain two segments that cannot talk to each other.

[–] marauding_gibberish142@lemmy.dbzer0.com 2 points 3 weeks ago (1 children)

Thank you so much for the explanation. I followed everything but:

Untagged (sometimes called Access) is something you apply on a switch port. For example, if you assign a port to Untagged VLAN 32, anything connected to that port will only be able to connect to port 32.

I couldn't really understand what you meant here. Did you mean VLAN 32 in the last line?

[–] neidu3@sh.itjust.works 1 points 3 weeks ago* (last edited 3 weeks ago)

Derp, yes. Corrected. VLAN numbers are obviously not related to port numbers in any way.