IP Addresses are used to identify computers and devices on a network, each device on a network will have its own unique IP address. IP Addresses can be thought of as a physical postal address, they have to be unique so that the postman knows where to deliver the post.
A dynamic IP address is an address that can, and will, change when it expires. Dynamic IP addresses are allocated by a DHCP server. The job of the DHCP server is to allocate dynamic IPs to devices on the network that do not have IP addresses so that they can send and receive messages. A Dynamic IP address will have a lease time, and when the lease time is up the IP address will expire and be available to be reallocated by the DHCP server. This means that a computer will have to renew their IP address or receive a different IP. Expiration of IP addresses mean that if you have a computer on your network that you decommission, you sell it or remove it from the network, the dynamic IP address that it received will eventually be released and available for another computer on the network.
Static IP addresses are IP addresses that have been manually set and will never expire unless manually removed. Static IP addresses are not allocated by a DHCP server, they are manually set on the computer. Static IPs are useful for a computer whose address you do not want to change, such as a server. Usually, computers that need to talk to servers on the network will store the IP address of the server locally on the machine. This means if the server has a dynamic IP the locally stored IP address will have to change whenever the DHCP server renews or changes the server’s IP. If the server is given a static IP address then the client computer won’t have to update the locally stored IP address of the server, as it will never change.
On the internet, web servers also make use of static IP addresses. This means when you type in “www.google.com” the resolved IP address will be a static IP address. When the local DNS resolves the IP address it will cache it locally, and so will your computer, which means if the webserver that hosts “www.google.com” changes it’s IP address then the page will not display correctly. This is because when you next visit the website your computer will use the locally cached IP address of the website, so if the IP has changed you may get sent to a different website or the website will be unreachable. However, because web servers on the internet use static IP addresses this problem never occurs.