I'm working on a Cisco 2621 router. It has four Serial ports and two Ethernet ports. How can a Serial port be converted to an Ethernet port?
Perhaps there is some aspect of your question or of your requirements that I am not understanding. But there is not any way to convert a Serial port into an Ethernet port. You may look at the connectors and see that physically they take similar connectors. But they employ very different protocols and use very different electronics. If you need more Ethernet connections then it should be possible to get an additional Ethernet card for the 2621. But you can not convert Serial into Ethernet.