It doesn't register with the DNS when it's dynamically booted.
Dynamic DNS registration of DHCP booted things is a relatively new invention. Not all DNS servers support it, and some setups assume that the DHCP server is going to update the DNS, rather than the DHCP client.
If you want a known name associated with the empeg, get the DHCP server to always hand out the same address (using a hardware address mapping), and then configure the DNS server with the name you'd like.
Roger - not necessarily speaking for empeg
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roger