Today I was using tcpdump and I noticed my computer was having IPv6 traffic with a particular MAC address that I could not match with an IP using nmap or arping. After looking at the tcpdump logs a little more closely, I figured out it was another MAC address my router was using but exclusively for IPv6 traffic. 22:49:01.936830 90:0d:cb:ff:31:91 (oui Unknown) > 33:33:00:00:00:01 (oui Unknown), ethertype IPv6 (0x86dd), length 158: fe80::920d:cbff:feff:3191 > ip6-allnodes: ICMP6, router advertisement, length 104 Why would a router offer a different MAC address for IPv6?