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According to the notes I'm reading:
10/100 Mbit/s Ethernet refers to the standard that can autosense which speed it needs to run between speeds of 10 Mbit/s or 100 Mbit/s.
Why would autosensing be required? Wouldn't it be best to result to 100 Mbit/s or will this impact the network in a negative way?
Some devices could only run at 10 megabit/s, so the device at the other end would autosense the speed to match. If a device that has a maximum speed of 10 Mbit/s is connected to a 10 Mbit/s / 100 Mbit/s switch, the switch needs to lower its speed on that particular port in order to effectively (efficiently) communicate with the device.
These days, most devices will autosense between 10 Mbit/s, 100 Mbit/s, and 1000 Mbit/s, but back in the days of "fast Ethernet" the choices were 10 Mbit/s and 100 Mbit/s.
Remember 10Mbps came first, then 100Mbps, then 1000Mbps.
The advantage of supporting multiple speeds and automatically switching between them is you can upgrade your network gradually without having to worry about what speed each device supports or replacing everything at once. You just plug a device in and it connects at the highest mutually supported speed. There are a few devices that are single speed but they are uncommon enough that it's not usually too much of a problem (though you do come across the occasional person wondering why thier Raspberry Pi won't talk to their media converter).
I think this is a large part of the reason twisted pair Ethernet has remained the dominant networking technology. It has been able to massively increase performance while keeping (almost) everything compatible.
Unfortunately with 10G the waters have got a lot muddier, afaict no devices support both 10M and 10G and some older ones don't even support 100M. I imagine the situation will be similar with the new 2.5G and 5G stuff. Fortunately newer 10G devices tend to support 100M and most 10M devices are probablly out of serivce by now so hopefully the breakage shouldn't be too painful.
10/100 autosensing is for backwards compatibility with old hosts that are 10M only.
10M was the only ethernet for a long time. When 100M was introduced, it was extremely expensive and only used for backbones or servers (and not all servers either, just the important one or two that could justify the cost). Therefore it was assumed that 10M devices would be around for a long, long time.
Interestingly enough: 10/100M NICs were so expensive that using one to talk to a 10M device was initially considered wasteful. Why spend all that money to talk to a 10M device unless that 10M device was scheduled to be upgraded. If it was decided NOT to upgrade
Why was 100M either so expensive initially? Besides the fact that new tech is usually more expensive at first, at the time the only other network technology that was 100M/s was FDDI which was extremely expensive and extremely difficult to manage. So, the 100M/s ethernet vendors could charge high prices and it would still be cheaper than FDDI. If bread cost $100/loaf and you found a way to make it for $10/loaf, you'd charge $99/loaf for as long as you could, right?
I remember a Cisco salesperson in 1996-ish saying "we expect 10/100 switch ports to drop to $100/port eventually, so in the future we won't offer 10M... just 10/100M." At the time this seemed like crazytalk.
It's all obsolete, in the current era. Which means whatever you are reading is obsolete.
Anything less than 1000Mb/s is ancient history. Fortunately for folks with antique equipment still running, myself included, most 1000 Mb/s ports will autonegotiate down to whatever the obsolete equipment is running at.
The point of autonegotiating is being able to speak to an obsolete device at a speed it can handle, without having to specifically configure a port to talk that slowly.
