Are you trying to use the /usr/sbin/arp command?

Since the Classical Internet Protocol (IP) network model resolves IP-to-ATM address pairs rather than IP to MAC address pairs, the /usr/sbin/arp command does not support Classical IP interfaces at this time. A version of the arp command, /etc/opt/SUNWconn/atm/bin/atmarp, provides similar functionality for Classical IP interfaces. Refer to the atmarp (1M) man page for more information.