Ubuntu can be installed using other methods as well, which might prove handy in some circumstances. For example, let's say you want to install Ubuntu on a computer that has no CD/DVD-ROM drive.
Read it in this blog
Install Ubuntu from a network server
For this task, you will need another computer that has Ubuntu installed (and working) and it's connected to the machine you want to install Ubuntu, through a LAN (Local Area Network). We will call that PC that already has Ubuntu, “Server”,
and the PC on which you want to install Ubuntu, “Client”.