configuring IPv6 networking in Ubuntu server

First backup current network configuration
#cp /etc/network/interfaces /etc/network/interfaces.backup

How to delete an ipv6 address :  #ifconfig eth0 inet6 del 2604:2881::8fe5:27e2/64

How to add an ipv6 address :  #ifconfig eth0 inet6 add 2604:2881::8fe5:27e2/64

Add an IPv6 route through gateway

#route -A inet6 add 2604:2880::8fe5:27e2/64 gw <gateway ip>
OR
#ip -6 route add 2604:2880::8fe5:27e2/64 via <gateway ip>

#/etc/init.d/networking restart

to see the new ipv6 address :
#ip -6 address show eth0

to see ipv6 route :
# ip -6 route show dev eth0

PERMANENTLY ADDING IPV6

Append the below lines in #vi /etc/network/interfaces

iface eth0 inet6 static
pre-up modprobe ipv6
address 2604:2880::8fe5:27e2/64
netmask 64
gateway 2247:f0d0:2001:000a:f0d0:2001::1
dns-nameservers 2001:4860:4860::8888 2001:4860:4860::8844

If ipv6 nameserver IPs are not there in /etc/resolv.conf then add Google Public DNS IPv6 addresses

nameserver 2001:4860:4860::8888
nameserver 2001:4860:4860::8844

#route -A inet6 add 2604:2880::8fe5:27e2/64 gw 2247:f0d0:2001:000a:f0d0:2001::1

#/etc/init.d/networking restart

To test ipv6 connectivity :

#ping6 google.com