Since the Amazon Echo will only send IFTTT commands to public IP addresses, even if the Echo is on the same Wi-Fi network as your ARC device, you will need to setup port forwarding on your router to send the port assigned to your HTTP server.

The setting is router dependent and can usually be found in the firewall section. I recommend forwarding both TCP and UDP ports

Optional: If your router is subject to frequent public IP address changes, consider a free service like DnyDns.org to provide a public name for your IP address