I have a Linksys WRT54GSv4 Router running DD-WRT connected to a Paradyne 6211-I1 (now Zhone) ADSL modem. My connection is pure DHCP. The router gets a public IP just by turning it on.
I wanted to access the modem config pages to view the sync rates without having to plug my computer directly in to the modem. This way I had real numbers to base my QOS settings on.
The dd-wrt wiki Talk:Access To Modem Configuration helped me and is based on Access To Modem Configuration
In my case I changed the IP of my router to 192.168.2.1 so the modem could still be accessed at 192.168.1.1 even if reset to factory defaults.
Go to Administration -> Commands
In the Edit box enter:
ifconfig vlan1:0 192.168.1.2
/usr/sbin/iptables -I POSTROUTING -t nat -o vlan1 -d 192.168.1.1 -j MASQUERADE
Click "Save Firewall"
Then "Reboot" from the Management page
IP summary for above commands.
Modem IP: 192.168.1.1
Router IP: 192.168.2.1
Go to these IPs for the configuration pages.
Sometimes I would have access to my neighbors' modems as arp requests were repeated by my ISP's DSLAM.
arp -s 192.168.1.1 MODEM_MAC_ADDRESS_HERE
fixed the issue.
Note. Replace MODEM_MAC_ADDRESS_HERE with the MAC address of your modem