Just thought I would recount my experience in case it would help others. For quite some weeks my router log was showing lots of entries like this:-
2006.10.14 23:26:11 **Smurf** 184.108.40.206->> 192.168.1.3, Type:3, Code:3 (from LAN Outbound)
2006.10.14 23:18:45 **Smurf** 220.127.116.11->> 192.168.1.3, Type:3, Code:3 (from LAN Outbound)
2006.10.14 23:11:20 **Smurf** 18.104.22.168->> 192.168.1.3, Type:3, Code:3 (from LAN Outbound)
PlusNet tech guys could not really offer much help. In my many attempts to fix, I also noticed I could not access www.3com.co.uk web site - or any other 3com website. All other sites seemed fine. I was trying to get to 3com website to see if they could assist in any way with my problem - my router is a 3 com model. The strange thing was that a Toshiba Laptop and Apple Mac using the same router could access the 3com websites. All very strange! I should point out that the 192.168.1.3 IP address was the computer I was having difficulties with connecting to the 3com website. I have tried all sorts of things to try and correct DNS problem with 3com - plus attempts to stop the smurf DoS. These attempts included disconnecting the router and powering down and up also - all to no effect. Looked for funny hosts files, corrupted DLLs, got rid of an index.dat file, switched off hardware and software firewalls, etc.
This morning I decided to configure the router so that it started it's IP addresses a lot higher than the standard start of 192.168.1.3. I then found I had to release and renew the IP addresses for each machine - except the Mac. Once having done that, I now have no more smurf attacks and can connect to 3com websites - not that I need to now! During the process, I noticed the router disconnected and reconnected itself along the way a few times.
Hope this helps others who may suffer the same experience.
Guess what? Symptoms have returned after a few weeks of everything being OK - the only difference being that the smurf attack was going to my new IP adddress that I had setup - same computer. Again, could not access www.3com.co.uk. This time, I just released the IP address and then renewed it and lo and behold it fixed it. Still don't understand what it is that causes the problem - and don't really understand how/why it fixes it - but hey who cares!?