clarrification on "agentaddress" in snmpd.conf

Is this directive the same as specifying the listening address(es)
in the command line?

I've tried the following:
  ./snmpd udp6:161,udp:161

And it works ok. The agent is listening on ipv4 and ipv6 udp port 161.


I then added "agentaddress" in snmpd.conf with the same
address list:

--
agentaddress udp6:161,udp:161
--

Then run snmpd:
 ./snmpd -c snmpd.conf

But I got the following error:
 
Error opening specified endpoint "udp:161"
Server Exiting with code 1


Whats exactly the use of agentaddress directive? I'd like snmpd to run
on udp6:161 and udp:161 and specify them from the config file(snmpd.conf?)
and not from the command line. Is this possible?

Thanks.