iMac Max Ethernet speed

Assuming the specific router/switch box involved here is 1000 Mbps gigabit-capable (check that)...

Swap the existing Ethernet patch cable for new or known-good Cat6-rated cable...

As you’re potentially either using an older Cat 3-rated cable (for 10 Mpbs Ethernet 10BASE-T), or the Cat-5e or Cat-6 cable you’re using is damaged and falling back to 100 Mbps Fast Ethernet speeds, or the cable is mis-wired.

I wouldn’t use any Ethernet cabling rated below Cat 6 these days.

Cat 8? FWIW, anything past Cat 6 is unnecessary with Gigabit Ethernet.

Cat 6a cabling is nice to have, if you’re planning to maybe eventually upgrade to 10 GbE. But Cat 6 will work with 10 GbE out to ~55m.

Cat 8 is intended for server racks and data centers, and part of 25 GbE and 40 GbE networking, and some similarly expensive local switching.

Apple has no Mac available with support for 25 GbE.

Only a few of Macs support 10 GbE, and this iMac Late 2009 is not among those: Use 10Gb Ethernet with your Mac - Apple Support

The link speed on Network Utility says 100 MBPS.

The other thread says that model has 1GB ethernet, which is 1000MBps. The MacTracker database confirms that:

Why do you think you have only 100MBps? iMacs have enjoyed 1000MBps since 2005.

What speed does your provider claim to offer?

If I may simply add that if there are other devices connected to the network that only support 10/100 Mbps, then that will be the internal link speed. To get full Gigabit Ethernet, all devices on the network must also support it. Also, using the proper cables is also important.

I’fe also occasionally seen bent pins in and debris in Ethernet sockets, and that can have the same effect as a bad patch cable.

Don’t conflate your ISP-claimed uplink speeds with the hardware speeds using the installed hardware.

I and others have also encountered cases with lower-spec hardware provided by the ISP or another entity, too.

And encountered simply mis-wired or failed hardware.

When in doubt, swap for a known-good Cat 6.

Only by purchasing a newer computer that supports GigaBit Ethernet.

Agreed.

You have been advised correctly, if you want gigabit speeds you need a new Mac that supports it.

Be careful on units notation too, as 100 MBps (commonly megabytes per second) is roughly 1000 Mbps (commonly megabits per second), depending on who’s using the units and who is using which notation.