Rails server with -b option failing on Linux

Ruby on Rails
1

I’m running Kubuntu 13.10 64-bit, recently installed fresh; was previously running Kubuntu 12.04.

I’m trying to start my rails server on another IP address than my local machine, say:

   rails server -b10.12.12.99

which I know is not in use (nmap says “Host seems down”). Yet I get an ADDRNOTAVAIL error from Webrick:

   => Rails 3.2.13 application starting in development on http://10.12.12.99:3000
   ...
   Cannot assign requested address - bind(2) (Errno::EADDRNOTAVAIL)

This worked on my previous installation. I don’t have a software firewall running. Any suggestions for resolving this?