I recently purchased a new PS3 as my old one finally went kaput (RIP). However, when I tried to sign in to the network so I could download all of my games/packs, I would receive "Network Unavailable".
Here's what I had done previously:
I had my new PS3 hooked up via an ethernet cable, and could confirm that the internet browser does work and it is able to download all updates for games/hardware.
I had verified that my media servers were disabled, I had verified that the date/time was set to automatically detect, and I had power cycled the modem/router as well as the PS3 itself. All other devices in the house were working fine on the internet connection.
Here's how I fixed it:
Rather than trying to sign in to PSPlus or the PSStore, I went to PSHome - it allowed me to input my username/password. Once I had signed in, I was then able to sign in to PSPlus and the PSStore.
I hope this helps other people who are still having the same issue.