Good day!

I have a 2500 series router and wish to connect to my modem. The serial on the 2500 series is a DB60 and the modem is an RJ45 therfore I am presuming that I will need a male DB60 to female RJ45. Is this correct? Will this allow my 2500 series router to access the WAN? Is there anything that I am missing or should know about? I am new to the Cisco world and currently working towards my CCNA so please excuse me if I'm off track. Any help will be greatly appreciated!