I just figured out (accidentally) how to boot into the Grub menu, but I didn't know what commands to enter to do anything else. I understand that's how you boot into an earlier kernel.
I was trying to get into my BIOS so I could disable Secure Boot, which I did.
Would booting into an earlier kernel allow me to install the driver, then it would work if I booted back into the later kernel?
I think it is the problem.
The drive is currently formatted in NTFS and still contains the files it had under Windows.
I was planning to upload the files to my cloud server, then format the drive as Ext4 so the OS recognizes it as an internal drive and allows full access. But I wanted to get the Ethernet issue fixed first, as there's a lot of data on that drive and it would take a long time under a slower connection (150 mbps on my wireless).
I hope I haven't missed anyone with this general reply:
I disabled Secure Boot this morning.
Now my 2.5 gb Ethernet card is available in the Network settings again, but I'm back to 1000 mbps speed.
I haven't made any further attempt to install the driver yet. If I understand what's happening correctly, I would still get the kernel conflict error and my Ethernet card would "disappear" again. But I'm willing to try it.
I figured out how to boot into Grub (by accident), so I could try booting into an earlier kernel. I would just need to know what commands to enter. If I boot to an earlier kernel like Zorin 17, install the driver and it works, will it work when I boot back to Zorin 18?
When You are in the GRUB Menu, choose the Option ''Advanced Options'' and there You choose the older Kernel to start.
I booted to the oldest kernel available, 6.14.0-33
Tried installing the driver file, same result - errors galore, Ethernet options disappeared in the graphical interface, came back when I rebooted. Still at 1000 mbps.
My Ethernet card is Realtek which seems to create a lot of problems in linux. I had to uninstall the latest driver (for me it was r8169) and replace with r8168-dkms. That solved the throttling problem (which was more like 1gig going down to less than 100Meg) but I had problems with both my sound card and my ethernet card which were solved by using 6.8 kernel. (I'm in 17.3 pro). Did you look in mainline to see what kernels were available?
I have the same realtek ethernet chip as you. I installed the driver that was mentioned as well. I also didn't see ethX as the driver readme said to check. However, I am using a cat6 cable and not cat8 as you mentioned.
My symptoms were also a little different, I had a flapping interface prior to driver installation. watch ip a showed the interface going down and up every second. After running the realtek driver install script, it showed 2500mbps and ceased flapping.
I'm curious if you can see flapping too, but maybe from down to up 1000mpbs to up 2500mbps or something similar.
I'm on z18pro and still a newbie at linux but I hope this helps in some way.
My Ethernet card is also Realtek and the driver is also r8169. I'm looking up r8168-dkms. I think I found a download link but I want to look into the kernel first. I did try 8168 first, I downloaded it from Realtek.
I'm running Zorin 18 kernel 6.14.0-33~24.04.1
I installed Mainline and it looks like there are a lot of kernels available, going down to 6.0 I don't know which kernel to choose at this point. I'm still doing research. I didn't know what Mainline was until I looked it up.
I may just buy a 2.5 gb USB to Ethernet adapter that's plug and play and compatible with Linux, and be done with it.
That is the easiest way to solve the problem you are facing, and you get my full support! Let me help you pick one out.
What I like about this one, its using USB-3.0, so it makes sure it can actually handle 2.5gbps, it has the quick glance activity lights, which is a quick way of confirming proper connections, has an aluminum case for heat dissipation, and I feel that its aesthetically pleasing as well.
@Onashi I may have missed it earlier, but are you dual-booting alongside Windows?
If you are, go to Windows>Settings>Power and disable "Fast Start-up" checkbox.
With that disabled, Windows should not hold on to your hardware on shutdown, as shutdown with Fast Start-up enabled is a form of hibernation and can affect devices when booting another OS.
Try 6.1 because this is an LTS Kernel. So, it gets Support. Or You go in the different Direction and choose the newest stable Kernel which would be 6.17.
I looked at that one, but Amazon Canada was going to take a week to ship it to me.
So I ordered this one. The description explicitly states that it doesn't require drivers (which I'll believe when I see) and a couple of reviewers said it worked with their Linux without installing a driver. I can use it with the USB-C port on my PC. It arrives later today.
https://www.amazon.ca/dp/B0D3FM7Z4L?psc=1&smid=A1JF9937NTN50K&ref_=chk_typ_imgToDp
" To resolve issues with the RTL8111/8168/8211/8411 PCI Express Gigabit Ethernet Controller on Ubuntu 24.04, the recommended approach involves installing the official Realtek driver, as the default r8169 driver may cause performance problems such as capped upload speeds. The r8168 driver from Realtek is known to provide better performance and stability.
Begin by removing any previously installed r8168-dkms package if present:
sudo apt purge r8168-dkms
Install the necessary build dependencies:
sudo apt-get install build-essential linux-headers-$(uname -r)
Blacklist the default r8169 driver to prevent conflicts:
echo "blacklist r8169" | sudo tee -a /etc/modprobe.d/blacklist.conf
Download the official Realtek GBE Ethernet driver for kernel up to 6.8 from Realtek's website or a trusted mirror.
Extract and install it using the provided script:
cd /path/to/downloaded/driver
tar xfvj r8168-8.054.00.bz2
cd r8168-8.054.00
sudo ./autorun.sh
Verify the driver is loaded correctly using:
ethtool -i <your_interface_name>
Replace <your_interface_name> with the actual network interface (e.g., enp6s0 ).
Reboot the system to ensure the changes take effect. Note that the driver must be reinstalled after each kernel update, as it is not automatically updated with the system.
Alternatively, a community-maintained GitHub repository provides a stable version of the driver (8.047.05) that is compatible with Linux 5.3 and later.
This can be cloned and installed similarly:
git clone https://github.com/airium/Realtek-PCIe-GBE-NIC-Driver.git
cd Realtek-PCIe-GBE-NIC-Driver/r8168-8.047.05
sh autorun.sh
After installation, confirm the driver is active using ethtool -i .
The official Realtek driver resolves issues such as sending rate limitations under TCP-BBR. AI-generated answer. Please verify critical facts."
I am guessing that you are on Zorin 18. Zorin 17 is on 6.8.0-87-generic kernel. I don't know if Zorin 18 (fork of Ubuntu 24.04) works with an earlier kernel.
Progress report for today - no progress to report. 
swarfendor437 - thank you very much for the information, it's actually very helpful. It's nice to get explicit instructions as to which commands to enter, instead of being told "do this" but not how to do it. (I'm new at this)
But - I did some more research on that Ethernet card (the integrated card that came with my PC) and found its top speed is only 1000 mb/s. Which is why I bought the new 2.5 gb card, which isn't getting full 2000 speed because of the driver (kernel conflict?) I did try your instructions but got the same result, kernel conflict.
I had previously tried to install the driver for the 2.5 gb card, but at the time all I knew to do was follow the inadequate instructions in the Readme file that came with the driver. I found the correct driver on the Realtek site, r8125. But it's only compatible with kernels up to 6.12. I'm currently on 6.14.0-35
Once (if) I get the kernel stuff sorted out, I will follow your instructions to install the driver for the 2.5 gb card.
Ponce-de-Leon - I tried using Mainline to downgrade the kernel to 6.1.11 but it won't boot to an operating system under that kernel, even under recovery mode.
Re the USB to Ethernet adapter - it's detected and it works but... 1000 gb speed again, needs a Realtek driver, same old same old, I found the driver but it's only compatible with kernel up to 6.10.
Update:
I believe I have the driver installed for the new 2.5 gb Ethernet card. However, the speed is still stuck at 1000 mbps. Now I'll have to figure out how/if I can configure the driver.
I followed the instructions below:
sudo apt-get install linux-headers-$(uname -r)
sudo apt install devscripts debmake debhelper build-essential dkms
Next, clone the Realtek RTL8125 driver repository:
git clone https://github.com/awesometic/realtek-r8125-dkms.git
cd realtek-r8125-dkms
Run the installation script to compile the driver and integrate it with DKMS:
sudo ./dkms-install.sh
Then I rebooted.
I ran lspci -vv and got this output:
> 3:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8125 2.5GbE Controller (rev 04)
> Subsystem: Realtek Semiconductor Co., Ltd. RTL8125 2.5GbE Controller
> Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx+
> Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
> Latency: 0, Cache Line Size: 64 bytes
> Interrupt: pin A routed to IRQ 18
> IOMMU group: 16
> Region 0: I/O ports at 4000 [size=256]
> Region 2: Memory at a4400000 (64-bit, non-prefetchable) [size=64K]
> Region 4: Memory at a4410000 (64-bit, non-prefetchable) [size=16K]
> Capabilities: <access denied>
> Kernel driver in use: r8125
> Kernel modules: r8125This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.