I've just installed it on my cacti 0.8.7b with plugin architecture latest version PA 2.1 on a CentOS 5.2 x86...
The problem is that I can't find it anywhere in the webinterface to approve it.
I've placed it even right under the settings plugin but without luck.
With PA I have a plugin management page as well but it doesn't show up even there.
It is a very simple plugin (encapsulation of the ipsubnet-body.html) and dont use all the PA2.x feature at this time. But it functions, I am using PA2.1.
1/ do you have set in /plugins/ipsubnet/config.php at least one of the parameters to TRUE. If you only want only ipsubnet_menu = TRUE (at left) you need a user with console rights
2/ If you have a old release you could have a realm conflict with a other plugin or have tab_ipsubnet.GIF instead tab_ipsubnet.gif under /plugins/ipsubnet/images
3/ could you open with an editor the .php and .html files and check if you have the (unix) carriage return. (I have uploaded the last time from a windows server)
4/ any rights problem in /plugins/ipsubnet/ or the files in it ? (for the same reason as 3/)