How to install a loopback interface in Linux for GNS3.

After successful installation of GNS3, we will install loopback adapter on our Ubuntu, Centos, Redhat and Fedora systems, so that we can telnet into your routers.

Loopback tap installation on Ubuntu
user@admin-desktop:~$ sudo –i
root@ admin-desktop:~#apt-get install uml-utilities
root@ admin-desktop:~#modprobe tun
root@ admin-desktop:~#tunctl             This will create loopback interface tap0
root@ admin-desktop:~#ifconfig tap0 10.100.100.100 netmask 255.255.255.0 up
root@ admin-desktop:~#ifconfig            verify that tap0 is up and given ip is assigned.


If you want to add one more loopback interface
root@ admin-desktop:~#tunctl             This will create loopback interface tap1
root@ admin-desktop:~#ifconfig tap1 10.100.101.100 netmask 255.255.255.0 up







source: http://networkingtips-tricks.blogspot.ru/2010/09/how-to-install-loopback-interface-in.html