A webmaster at Google Groups may have uncovered an issue with a specific firewall program named CSF (ConfigServer Security and Firewall). In short, this webmaster found that his host used this firewall and it was blocking GoogleBot from accessing his web pages.
If you are noticing that GoogleBot cannot access your site, check to see your firewall. If any of the GoogleBot IPs are listed in the "Firewall Deny IP's" area, then remove them asap. The webmaster documented some of those current IP ranges (note, they may change):
- 18.104.22.168/19 # Googlebot
- 22.214.171.124/19 # Googlebot
- 126.96.36.199/18 # Googlebot
- 188.8.131.52/17 # Googlebot
- 184.108.40.206/20 # Googlebot
- 220.127.116.11/16 # Googlebot
- 18.104.22.168/19 #Googlebot
Make sure to verify those IPs are GoogleBot before preceding yourself.
Then add the range of Googlebot IP addresses to bot the FIREWALL ALLOW IP'S and the LFD IGNORE IP'S in the firewall setup.
It is important that you regularly check Google Webmaster Tools and/or your log files for accessibility issues. You can be missing traffic and not even know about it.
Forum discussion at Google Groups.