Static and dynamic IP address

A static IP address is one that remains fixed and never changes. The PC always sees the same address regardless if the session between the PC and ISP is broken or not. A Dynamic IP Address is one that changes over time or in the case of a Dial-up connection, when the user disconnects and reconnects to his/her ISP.

Static IP addresses

Static IP addresses are manually assigned to a computer by an administrator or your ISP. The exact procedure varies according to platform and the company that provides your Internet connection to you. Usually, if you have a business connection for your corporation or you are connecting a server for your office, you would be allocated one or more static IP addresses for your business.
These IP addresses are programmed on your routers and other network equipment supplied by your ISP or purchased by you.

Dynamic IP address

This contrasts with dynamic IP addresses, which is assigned either randomly, or arbitrarily assigned by a server using Dynamic Host Configuration Protocol (DHCP). Even though IP addresses assigned using DHCP may stay the same for long periods of time, they can generally change. In some cases, a network administrator may implement dynamically assigned static IP addresses. In this case, a DHCP server is used, but it is specifically configured to always assign the same IP address to a particular computer, and never to assign that IP address to another computer. This allows static IP addresses to be configured in one place, without having to specifically configure each computer on the network in a different way.

Uses of static addressing

Static addressing is essential in some infrastructure situations, such as finding the Domain Name Service directory host that will translate domain names to numbers (IP addresses). Static addresses are also convenient, but not absolutely necessary, to locate servers inside an enterprise. An address obtained from a DNS server comes with a time to live, or caching time, after which it should be looked up to confirm that it has not changed. Even static IP addresses do change as a result of network administration.

Uses of dynamic addressing

Dynamic IP Addresses are most frequently assigned on LANs and broadband networks by Dynamic Host Configuration Protocol (DHCP) servers. As a rule of thumb, if you have a regular home Internet connection with any Dialup, Cable or DSL provider, you will be getting a dynamically allocated IP address. In some cases, these addresses will be more or less constant as it it often observed with Cable networks, but they are not guaranteed to always remain the same and can change at any moment. If you are planning on hosting any servers on your local home network, you would be best to obtain a static IP address, to guarantee availability of your server. Dynamic IP addresses are used because it avoids the administrative burden of assigning specific static addresses to each user on a network. It also allows many users to share limited number of IP addresses available on your ISP if only some of the users will be online at a particular time. In most current desktop operating systems, dynamic IP configuration is enabled by default so that a user does not need to manually enter any settings to connect to a network with a DHCP server. DHCP is not the only technology used to assigning dynamic IP addresses. Dialup and some broadband networks use dynamic address features of the Point-to-Point Protocol.