For Centos 5.5 installation on GNS3 you need to create centos 5.5 VM on virtual Box, for this you can visit Createlinux virtual machine in Virtual Box.
After creating the VM click on start button to start VM (virtual machine). Press enter for starting setup.
Centos 5.5 setup start loading the OS installation start up files.
After completions of loading, setup ask you for checking installation media (DVD), you can skip this process or check the installation media for errors. I simply skip this process.
Click next on welcome screen.
In next step you need to choose partition for your HD, just select the default setting click on next it will asked you for full format click on yes.
In network setting you can choose DHCP for automatic IP or use manage IP setting according to your requirements. I assign manual IP setting for my VM, you can also change these settings later.
Also I didn’t assign any gateway setting for my VM that way this is showing this following error. I just ignore this message. If you want to complete you VM’s IP setting then provide the gateway and DNS.
Select your time zone.
Provide root password, root is built in user in centos like administrator in windows.
Select required packages for your OS according to your requirements and you can also customize these packages; I have chosen server packages according to my requirements.
Click next for start installations.
Setup will start installing OS packages of Centos 5.5.
On completion of installation reboot your system.
In firewall setting enable your required protocols.
Set date and time.