No, it's not common practice to have the ability to set up a static IP address. Besides, it's beyond the abilities of the vast majority of Roku users to make use of such a feature. I'm not calling anyone dumb or stupid. But properly using static IP addresses requires some networking knowledge that most casual electronic device users have no knowledge of. As you say, a DHCP registration can accomplish almost the same thing, but even that is beyond the knowledge of most users. Ask someone what their MAC address is and you will get a blank stare.
Dan
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