asterisk users - Apr 2006 - SIP register question

I am trying to link an asterisk box up to a SIP server on the same 
subnet. The SIP server does not have a password (and is locked down by 
IP number 'allow'). How do I specify this on the register line?

Based on the documentation, the line looks like this:

register => user[:secret[:authuser]]@host[:port][/extension]


It looks like user@host is the minimum required. Is there anyway to 
specify a username of "null", or something?

Thanks in advance!
Steve