ISPs (Internet service providers) assign an IP address to each device on their network. The IP address can be static or dynamic.
Static IP Addresses
A static IP address is an IP address that always stays the same. If you have a web server, FTP server, or other Internet resource that must have an address that cannot change, you can get a static IP address from your ISP. A static IP address is usually more expensive than a dynamic IP address, and some ISPs do not supply static IP addresses. You must configure a static IP address manually.
Dynamic IP Addresses
A dynamic IP address is an IP address that an ISP lets you use temporarily. If a dynamic address is not in use, it can be automatically assigned to a different device. Dynamic IP addresses are assigned using either DHCP or PPPoE.
About DHCP
Dynamic Host Configuration Protocol (DHCP) is an Internet protocol that computers on a network use to get IP addresses and other information such as the default gateway. When you connect to the Internet, a computer configured as a DHCP server at the ISP automatically assigns you an IP address. It could be the same IP address you had before, or it could be a new one. When you close an Internet connection that uses a dynamic IP address, the ISP can assign that IP address to a different customer.
You can configure your Firebox as a DHCP server for networks behind the device. You assign a range of addresses for the DHCP server to use.
About PPPoE
Some ISPs assign IP addresses through Point-to-Point Protocol over Ethernet (PPPoE). PPPoE adds some of the features of Ethernet and PPP to a standard dial-up connection. This network protocol allows the ISP to use the billing, authentication, and security systems of their dial-up infrastructure with DSL modem and cable modem products.