If you want a static external IP address then this has to be arranged with your ISP.
Even if you do know the DHCP range, gateway address and name server addresses you can't just give yourself a static IP in this range.
First of all how do you know that someone else hasn't been assigned the address you are trying to use?
Also your ISP may well have a system in place where any device has to have been given an address by the DHCP server or it won't be allowed to connect to the network. Most ISP's that I know of have this sort of setup.
At the end of the day your ISP is in charge of all of the IP addresses that they own. It's up to them, not you, how they get assigned.
Your ISP will probably be happy to give you a static IP if you ask them, although they will probably charge you for it.