I was told that this configuration (which contains an error in DNS mask which is /23 instead of /24) works only because it's a SHARED bus. If it was a SWITCHED lan, the DNS responses from DNS to A wouldn't "be forwarded". Why?

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Basically A, in order to communicate with DNS, has to pass THROUGH the DG (default gateway) because A sees the DNS address in another subnet. On the other hand, the DNS sees A in his same subnet (since IP A is inside DNS /23) so DNS "talks" directly (dest macaddress is A instead of DG so it's a direct communication) with A WITHOUT passing through DG.

I was told that this only works because the lan is a shared bus. If it was a switched lan the DNS response wouldn't be forwarded(????) Why? What does it mean? What's the issue?

  • On the other side: superuser.com/questions/1812671/… Oct 15, 2023 at 18:40
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    Nov 14, 2023 at 18:14

1 Answer 1


Obsolete shared-medium or repeated Ethernet with hubs or switched doesn't matter.

Both /24 subnets run in the same L2 segment, and there's a gateway routing in between. /24 hosts use the gateway to reach the other /24 network. /23 hosts talk to them directly. Both methods work, even asymmetrically.

This setup is only useful for educational purposes or perhaps during an address migration, as using a gateway when you don’t really have to is inefficient.

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