exchnage 2013 having two cas server .how to nat from outside?

I have two cas server and two mbx srv  with win 2012 os -exchange 2013 cul1.

I have configured round robin for high availability for cas . My question is how natting is possible for  two servers if i am using roudrobin . I don't have two public ip .

can we configure windows nlb and do nating on the array ip.How its going to work because we cant create cas array in exchange 2013..

For round robin -my A record i create is mail.abc.com and point the same name to both cas. Internaly its working.So if i am creating WNLB what name i need to create...how its going to work?

thanks,