$(1)$ Explain why IP addresses are not portable. That is$,$ you cannot use a NCTU campus's IP address inside the NTHU campus. $(2)$ If you use a NCTU campus's IP address inside the NTHU campus to connect to a host in the NTU campus, what problems will you encounter? $(3)$ Explain why Ethernet MAC addresses are portable. That is$,$ you can use your NIC at any subnet without the need to change the MAC address that it uses. $(4)$ If IP addresses are flat like Ethernet MAC addresses, what problem$($s$)$ may occur for Internet routers?