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.
1. To set up a static route between the Router and another network, select a number from the Static Routing drop-down list. (A static route is a pre-determined pathway that network information must travel to reach a specific host or network.)
2. Enter the following data:
· Destination IP Address - The Destination IP Address is the address of the network or host to which you want to assign a static route.
· Subnet Mask - The Subnet Mask determines which portion of an IP address is the network portion, and which portion is the host portion.
· Gateway - This is the IP address of the gateway device that allows for contact between the Router and the network or host.
3. Depending on where the Destination IP Address is located, select LAN & Wireless or WAN from the Interface drop-down menu.
4. To save your changes, click the Apply button. To cancel your unsaved changes, click the Cancel button.