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Comment by lamontcg | Hacker News Reader

When I worked at Amazon we found two NICs with the same MAC address.

And that was a long time ago with a comparatively very small server count compared to today.

Someone's manufacturing database probably had a tiny race condition.

Worked for a small ISP in early 2000s. Not every new customer had an ethernet card in their PC and we had problems like installation crews calling the office that there is no port to stick the cable in.

The company obtained 300 cheap Intel PRO/100 Fast Ethernet NICs from China. The idea was that the crews would have a bunch of those cards to be able to install them in customers' computers.

Everything was dandy until second installation in the same building. For a little more backstory - it was a kind of shitshow infra with dumb switches and all customers in a building being in the same LAN.

So... the second installation in the same building rendered the former newest customer's link inoperable. Or wouldn't work, then suddenly there was connectivity, then there was no ping to the gateway but some other devices were pingable etc etc. The crew replaced the card with another one (from the same shipping), re-crimped RJ45s on both ends, changed switch port and nothing - same shit with random internet connectivity losses. Weird situation with seemingly no reason for what was happening.

It later turned out that the reason was that all 300 NICs had the same MAC address. All of them.

20-odd years ago, the college I attended (and worked for) was outfitting one of the computer labs with new network cards; migrating from Token Ring to Ethernet as I recall. To do this, 3COM cards had been bought by the boxful.

We replaced all of the cards, and booted the lab. 1 machine got an IP and then it all went to hell. Every single machine showed up with the same MAC. Think we had to rip and replace to fix it.

I used to work for a company that sold labels for various third party printer manufacturers, and let me tell you, some of them were VERY lazy when it came to assigning MAC addresses.

yep, not unheard of for cheap unbranded devices to all have the same MAC, unfortunately.

MACs are supposed to be globally unique, but they only really need to be broadcast domain unique, so you can get away with using the same MAC for a lot of devices as long as most people don't use two on one LAN.

I'm aware; that really doesn't make this excusable practice though.

I worked for a company that made a networking product and we managed to build 1,000 units with the same MAC address (we had a big block of assigned addresses) because of a miscommunication with the vendor who provided the preprogrammed microcontrollers..

Luckily the MAC was in a rewritable location so it was possible, if a royal pain in the ass, to reprogram them all.

I'm having trouble finding a reference just now, but the MAC address for a modem-based link on Windows had "DESK" until it started causing real trouble. And one of those problems was that GUIDs were obtained by using "the MAC address" of the PC as an input to a DEC-Intel-Xerox-Microsoft algorithm. Around the time of this coming to a head, it came up that some manufacturers were out of MAC addresses for whatever reason, and also that some manufacturers were assuming that two random NICs on an Ethernet would not both be one of $MANUFACTURER's cards, so care was not necessary.

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