We migrated a Plesk 8.3 server (Linux) to 11.0.9 (Linux) today. It mostly went fine, but we had two sites that would time out when trying to load in a browser window..Eight IPs total in the subnet. Seven were used and five had sites that were working fine.
So, I went to another server to do a telnet test and sure enough, I could telnet to it, but when I requested a page it would time out. These were both basic HTML sites. It was very perplexing.
The server we did the telnet test from was in the same datacenter as the new server. This was our mistake, because it turns out that three of the eight IPs in this block were null-routed from the abuse of a previous client. The null-route was handled at the upstream provider level, so our tests from another server in the same DC were invalid.
So, lesson learned, always test from a different datacenter!