Changing my ip and such

So I have airport extreme router and access it with my windows 7 computer. I can change the last bracket of my ip adress but cant change the third bracket

for ex 1.1.1.1 i can change to 1.1.1.6 but cant do 1.1.4.6. When I try to do this i get this " The router address you have entered is not compatible with your WAN IP address". I access this area through configure ipv4 manually. If it is really needed I can get the model number and such.