2N BRI Lite/Enterprise v1.1 - Manual (Page 50)

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MAC Address
The BRI gateway has a unique, factory-set MAC (Media Access Control) address. Refer
to the rear side label of your device or the gateway web interface for the MAC address.
The MAC address can be user-changed.
Tip
 If you use DHCP, you are advised to set permanent assignment of one and
the same IPv4 address to the defined MAC address to avoid unexpected
change of the gateway IPv4 address and subsequent VoIP-SIP setting
errors.