Error with the default Vagrant version in U14.04

root@alok-ubuntu-2:~/firstsetup/puppet-rjil# vagrant up etcd1
Bringing machine 'etcd1' up with 'virtualbox' provider...
There are errors in the configuration of this machine. Please fix
the following errors and try again:

vm:
* An IP is required for a private network.

root@alok-ubuntu-2:~/firstsetup/puppet-rjil# dpkg -l | grep vagrant
ii  vagrant                            1.4.3-1                               all          Tool for building and distributing virtualized development environments
root@alok-ubuntu-2:~/firstsetup/puppet-rjil#

Upgrading to 1.6.5 from https://www.vagrantup.com/downloads.html resolved this. 
Alternatively, we could just replace "dhcp" with :dhcp in Vagrantfile.