From the "use it or lose it" department....

I'm trying to use tcpdump to view *only* IPv6 traffic. The command I am using looks like this...
tcpdump -r filename 'ip[0] & 0xf0 = 6'

I am getting no results. Changing the 6 to a 4 to see IPv4 traffic also yields no results so I know I must have something wrong. Just to cover all the bases: yes, the file is a tcpdump file and yes I can see records if I leave the BPF filter off.