Okay, originally NetBeans was using the update URL:
http://updates.netbeans.org/netbeans...catalog.xml.gz
I changed it and downloaded the plugins.
My ethernet connection's subnet mask is 255.255.255.0. I set it in Network properties. I don't know why Netbeans is displaying it as 255.255.255.255.
The other router there is the school's wireless network, I think.
I'll try running the code again and let you know what happens. Thanks for your response.