What ports are in the rear of my iMac 21” Mid 2010?

Also

• Mini DisplayPort output with support for DVI, VGA, and dual-link DVI (adapters sold separately)
• Headphone/optical digital audio output (minijack)
• Audio line in/optical digital audio input (minijack)

For the Mini DisplayPort output, the maximum supported resolution is 2560x1600 pixels (which was the resolution of some old 30" monitors).

For connection information, see the following from: iMac (21.5-inch, Mid 2010) - Technical Specifications

Connections and expansion

• One FireWire 800 port; 7 watts
• Four USB 2.0 ports
• SDXC card slot

including a Gigabit Ethernet port.