Is it possible to have more than one subnet on single Ethernet, if yes why and how, also it is possible to have more than one Ethernet to share the same subnet, if yes why and how?
You can configure a secondary IP address on an Ethernet interface. I had to configure it recently on a remote router. The reason I did it was because I needed to access through a telnet session a device whose IP address was not in the proper range that was configured on the Ethernet interface. I configured the IP address range for the secondary Ethernet interface, next I created a telnet session to the router, and from the router I was able to telnet into the device and change its IP address.
In order to change the IP address to secondary, go into configuration mode and go into the Ethernet interface. Type the command "ip address x.x.x.x x.x.x.x secondary." After you type IP address you enter the IP address that you want to use and then you type the subnet mask. It is important to remember to type the word secondary at the end or the IP address will be the primary.