I use Dynamic DNS to be able to communicate to/from my laptop to the LAN. I recently noticed that when it was plugged in at home, the address it was updating was a DHCP issued by my router. Here is the fix I used to make it work correctly.
replace the use= line in your /etc/ddclient.conf with the following.
use=web, web=checkip.dyndns.org/
Sample /etc/ddclient.conf :
protocol=zoneedit1
use=if, if=eth0
server=dynamic.zoneedit.com
login='user'
password='password'
mydomain.com