Regarding the concept, check "Networking > Network Address Translation - NAT > Static Nat" at the menu bar above.
Regarding how you can do this in linksys, I can't tell you exactly since I'm not familiar with Linksys interface. It might have many names, like "one-to-one mapping", "IP address mapping", "Static NAT", however it should be pretty straight forward to find and configure -the manual should be of help.
Dsl modem is not a concern, as long as the public IP address is asigned to a router's interface the only concern is router's configuration.