What model Linksys router are you using? If it is the WRT54G or WRT54GS there is a company called Sveasoft
that has created modded firmware for those 2 models. With this firmware it gives you the ability to set Dynamic DHCP which gives you the option to set the same IP each time you reboot and or the router is reboot. The firmware also adds a lot more functionality to the router and I recommend that anyone with either model check out the Satori firmware. I am running it on my WRT54GS and it works great.