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What happens if a USB Ethernet Adapter has the same MAC address as the NIC?

Is it possible for internet connection to pass through the usb-ethernet adapter succesfully or not, in that scenario?

peterh
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Heaven88
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    I ask because the switch won't allow internet connection with the Ethernet USB adapter in the middle. From Switch to Usb Ethernet Adapter to Desktop PC – Heaven88 Nov 03 '18 at 06:38
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    Edit the question instead of adding information in the comments. – RalfFriedl Nov 03 '18 at 07:35
  • Also, you need to explain your first scenario more clearly: do you mean the MAC address of a built-in NIC? If so, your two scenarios are the same... – Ward - Trying Codidact Nov 03 '18 at 08:18
  • Some port replicators for laptops *do* set the same address to the NIC in the replicator as to the built-in NIC. In that case, you should link only the replicated NIC, not both. – Hagen von Eitzen Nov 03 '18 at 11:35
  • If you accidentally registered multiple accounts, you can merge them. – peterh Nov 04 '18 at 18:27
  • It can work, although it would cause only unneeded trouble (for example, if they are connected to the same LAN, you will have a MAC collision). What you really want, is probably a switch. It doesn't require having the same MAC, but it requires NICs which can send messages from any MAC address, and capable to listen the network in promiscouos mode. – peterh Nov 04 '18 at 18:30

2 Answers2

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A USB-to-Ethernet adapter is a NIC as well.

Two NICs must not have the same MAC address when connected to the same local network. If the network is spanned by a switch, its MAC table will be messed up by the constantly flapping source MAC address, causing random frame delivery to both NICs.

If they are connected to different networks (layer-2 segments) then it shouldn't matter - unless the uplink switch for both NICs is the same and it doesn't like seeing the same MAC in different VLANs.

Normally, this can't happen because a NIC's MAC address is almost always enbedded into its hardware. Assuming both NICs are from different vendors, the MAC addresses must differ in their OUI part (first three octets).

When using locally administered addresses (LAA) you need to take care not to use duplicates yourself.

Zac67
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If two hosts have the same MAC-address there are two problems: Number one is the aforementioned mixup in the switche's MAC-address table.

If you try to leave the network (through a router) you will get a problem within the router's ARP cache. The ARP cache connects an IP-address with a MAC-address. (check your PC's ARP cache out for yourself by typing 'arp -a' into the windows console.)

geets
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