Problem Symptom: The SonicWall keeps losing its dynamic IP address
First, verify your login credentials are correct.
Check that Ethernet cables and power connections are good.
For models with external power supplies, ensure the correct SonicWall power supply (5V, 2.4A rated: except TZ150 is 12V, 1.66A rated) is connected.
Power cycle the Cable/DSL Modem and the SonicWall. Power Off the modem and then the SonicWall. Power on the Modem First and then the SonicWall. Once the SonicWall completes its test phase, you should see transmission and reception lights start blinking on the WAN connection.
Test to see if the SonicWall received an IP address for WAN/DMZ. If it worked, then the Network > Interfaces (General > Network page in Firmware 6.x) page should now have filled in blanks for the WAN or DMZ interfaces (WAN/DMZ settings on Firmware 6.x). You should be able to ping known live addresses (or by resolving IP addresses of URLs).
Some ISPs use ping to determine if customer equipment is connected. By default, the SonicWall is not configured to respond to pings. To configure the WAN Interface to respond to ping:
Firmware 6.x: Enter the LAN IP address in the Public LAN Server box for the ping protocol on the Access>Services page.
SonicOS Standard: Use the Public Server Wizard to create the rule for Ping Service to the LAN IP.
SonicOS Enhanced: On WAN (X1) Interface General settings tab, enable the check box for Ping management feature.
Click System > Diagnostics. Choose Ping in the “Diagnostic utility” drop down in the Sonic OS Standard and Enhanced firmware. (In Firmware 6.x, Click Tools > Diagnostics).
Ping your ISP’s Default Gateway or any IP that is pingable on the Internet (e.g. 126.96.36.199).
If the Ping is alive, check the Default Gateway for the computers in the LAN. The default Gateway must be the SonicWall LAN IP.
Click Firewall > Access Rules > Disable any “Deny” rules from LAN to WAN. (Firmware 6.x Click Access - > Rules).
Ensure your ISP is not having connectivity problems. Try connecting with another firewall or directly with a PC.
Check your browser settings to make sure the proxy is turned off (if you do not have a proxy in your LAN) and you are connecting to the Internet through a LAN.
Make sure you don't have any open applications like VPN Client etc. Disable any Software Firewall or the WIN XP firewall.
Force the WAN Port speed to 10 Mbps Full Duplex. The WAN Port speed can be changed by clicking Network -> configure next to the WAN interface - > click Advanced and change the Link speed in the drop down (in the 6.x firmware Click Advanced -> Ethernet > Check the Force option in the WAN Link settings.
Click the Firewall > Access rules page, Edit the default LAN to * rule and check Allow Fragmented Packet (In the 6.x Firmware Access > Rules).