I have a truly bizarre problem, I can connect to: 217.10.79.23 but not to 217.10.79.28 -- Same mask, same network but somehow if the IP address ends with an even number, I can't connect to it.

Same goes for these:

can't connect:
217.10.79.28 www.sipgate.co.uk
66.35.250.150 slashdot.org
64.191.203.30 digg.com
85.234.146.50 my-bulldog-hell.co.uk

can connect:
217.10.79.23 sipgate.co.uk (this is a sip server, not the webpage)
64.233.167.99 google.com
72.14.205.83 gmail.com
66.180.174.35 distrowatch.org

Another clincher is if I restart my internet connection (PPPoA with bulldog) and get an IP address ending with an odd number (e.g. 87.74.78.35) then I can't connect to anything at all.

I have been doing traceroutes and I always reach the gateway but only reach the internet if the IP ends with an off number.

I think my ISP is responsible but they can't find a problem in almost 3 days so far...

Anyone seen something like this before and knows what could the problem be?

My thoughts:

You do not give any details of your setup (HW or Distro) either in your post or profile, so you are making diagnosis difficult.

But this may well be a hardware problem (you have checked your network & masks, and I used to do hardware). I suspect the LSBit of something is defaulting to 0 because it is not conected, or has failed for some reason:

Do you have another computer on your LAN? If not, borrow one. Give it an odd, then even IP address. Can you ping / connect? What if your IP address is odd / even. Try it again.
If errors here, then it's a local fault: Be methodical & isolate it, then change the hardware, and try again.

If the above fails, please try a different router, it may well not be your ISP's fault.

Out of curiosity, how are you connecting here? The IP of linuxquestions.org ends in an even number.