Newer CenturyLink modems typically have four Ethernet ports. Older modems may only have one Ethernet port and the port may be labeled DATA.
If your modem has more than one yellow Ethernet port, you can safely use any one of the yellow ports. The additional ports allow you to connect additional computers or peripheral equipment, like say a printer.
On newer CenturyLink modems the DSL or LINE port is color coded GREEN and it looks just like a phone jack. Older modem models may not be color coded.