Using the Windows DHCP server, you might noticed this message trying to do an IP reservation with the MAC address on your DHCP scope :
The unique Identifier you have entered may not be correct. Do you want to use this Identifier anyway?
And then :
The DHCP server received a message from a client that is not valid.
This is because you’ve entered the MAC address with colon ” : “
EXAMPLE
Instead of :
00:6d:3a:5e:23:12
You must enter it like this :
006d3a5e2312