Privacy is a major issue for 6LoWPAN networks and the use of persistent identifiers (MAC addresses) in the core mechanism is particularly challenging. Indeed, nodes use the SLAAC protocol in RPL to auto generate their IPv6 addresses based on their MAC addresses. Persistent addresses simplify the routing in the network but allow an adversary to analyze the traffic and recover sensitive information. We propose Ephemeral a MAC pseudonym scheme compliant with SLAAC. It provides dynamic pseudonyms cryptographically generated without the need to reconstruct the routing tables when the pseudonyms change. Our simulation based on CONTIKI 3.0 and WSNET shows that Ephemeral improves MT6D a previous MAC pseudonyms scheme by 16\% in term of application packet delivery. It also reduces the overhead of control frames introduced by MT6D.