Ethernet on new IMac 24

You need to purchase the 24" iMac that has the Ethernet connection on the power cord. If you purchased an entry level 24" please return it and buy the correct configuration which you can find at apple.com and select the mid tier option, you will note these are some of the models that have Ethernet. A great sweet spot is the 24" mid tier with 16GB of RAM, 1 TB SSD which is about $2099

Also, before you ask, to-date from what I have heard the power cord is not available as an after market accessory.

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Certain new iMac models have Ethernet on the power adapter. These are the "mid-tier" and "high-tier" configurations. It is also an option on the "low-tier" iMac. If you purchased one without Ethernet, you can use Wi-Fi (Connect your Mac to the internet using Wi-Fi - Apple Support) or use an adapter: The Belkin USB-C to Gigabit Ethernet Adapter - Apple.

Jack

If you got the low tier Mac with no connector, you can also get a dock with ethernet