i am 2000 pro user, and i am using a linksys wireless router G series on my desktop so that i may run my laptop , but here's my quandary.

1.) when i open up active ports 1.4 and i see my connections to yahoo or msn messenger i see that my IP number is 192.***.*.*** right ?
2.) but when i go to whatsmyip.com my IP shows as 68.***.**.**

i used the astericks as dummy numbers. my point is 1 IP starts with 192 and the other with 68
now the thing is i never encountered this when i was not using a router.

why do i have 2 different IPs with a router ?
==================================================================
but see this stems further into some high tech complicated stuff i wanna know about.
there's a program called SMAC which bascially allows you to spoof ur MAC ID number in order to obtain a new IP address once u reset ur network and modem, however since using the router I am unable to do that. would anyone know why ? would anyone know how to resolve that problem ? The reason I wanna change my IP address is for security purposes.
==================================================================