You want to get a DOCSIS 3 modem. I'd say you probably own your modem if you have had it for several years. I was in the same boat. You can buy or lease from Cox. My opinion is that leasing it is not good financially. I purchased my own modem. Cox has a page or suggested modems http://ww2.cox.com/residential/support/tv/article.cox?articleId=%7Bb2ec95d0-7ef9-11df-5590-000000000000%7D
I picked the one I wanted and shopped around for the best price, purchased it and then called Cox to activate.