Network Settings After Router reset

Question

My telephone switchboard had a fault (unknown to me) which made my ADSL Router interfere with my telephone. Initially I suspected that my router was faulty and went through a sequence of checks to isolate the problem. this included resetting the router to its factory settings. The switchboard problem having been fixed, the interference from the router ceased (the router was not the problem) and I proceeded to reconnect to my ISP but could not do so. The router gave solid DSL and Ethernet lights and I appeared to be connected but I could not open any pages in Safari. In desperation, I booted from my external hard drive; I was able to connect straight away and was able to reset my ADSL username and password. However, this only works on my external boot drive. When I re-boot using my internal drive I am unable to open any pages in Safari. It appears that something in my Network settings (?) on my main drive has been corrupted, but I cannot work out what (It is not Safari as that worked okay when I was booted from the external drive). Could it be my Network Preference Pane? How should I proceed?

Answer 1

Andy, many thanks for your input. I had already done what you suggested but tried it again. Diagnostics could not isolate the problem. The only difference in behaviour was that during the diagnostics process the 'Ethernet' light flashed at about 5 second intervals. Prior to using the diagnostics tool all 3 lights were steady and the Network preferences panel stated that I was connected to the Internet on built-in ethernet. When I open Safari, however, I get the dialogue box that I am not connected to the Internet and offered Network Diagnostics.
This is why I believe that my problem is a faulty Start-up item or extension (does OSX have 'extensions'?) but I do not know how to identify where the fault lies. I have searched my systems using Spotlight in an attempt to find a Network or Internet related file with a recent modification date but this has drawn a blank. Frankly, I think that there are just too many files for this approach to work. I guess that I can try replacing likely sounding files one by one, but I am afraid of making the problem worse by this approach.
Thanks again for your comments, but I remain open to any new approaches to solving my problem.