What is a static ip

A static IP address is a fixed IP address. Static. Unchanging. Always the same.

A dynamic IP is a variable or changeable IP address. Dynamic. Variable. The assigned IP address can change hourly, daily, monthly, when your gateway-firewall-router-modem device is power-cycled, or evem sometimes (but not with any certainty) not at all. Details here vary.

OS X Server — which is OS X with the Server.app installed — and most other servers will generally want a static IP address; a fixed and unchanging IP address.

OS X clients and most other client systems will accept dynamic IP addresses, and are generally configured to ask for a dynamic address. Client systems can accept and can use whatever IP address the DHCP server passes out. Using DHCP and dynamic addresses makes for far more efficient use of the available IP address space, particularly when mobile devices are in use.

The discussion of static and dynamic IP usually arises in the context of an ISP network connection. Residential ISP connections are usually dynamic addresses; DHCP issued, and variable. Secondary to this, ISPs variously also implement firewalls that block certain network activity from dynamic IP addresses; server-oriented network protocols can be blocked from the ISP residential dynamic IP service tier, and running web servers or mail servers are variously disallowed by the terms of service.

ISP static IP connections generally allow running servers, generally lack firewall blocks at the ISP, and the static IP address also means that DNS-based network checks will succeed; that other hosts will accept a mail server operating in the configuration, where other mail servers will often be configured to reject mail arriving from a mail server with a dynamic IP address. (Yes, it is possible for other hosts to determine this remotely.)

Generally, your ISP will charge you extra to get a static IP address. This is often associated with the ISP's business class service.

As MrHoffman has said, a mobile device will almost always get a dynamic IP address, as a static IP address must be used at a fixed location. Phones, laptops and tablets when the are out and about using the cellular network or as a guest on a public WiFi network.

Note, it is possible to give a device a fixed IP address behind your home router, for example 192.168.x.x or 10.x.x.x, as these are non-routing IP addresses and are addresses generally used in home networks and in a businesses private network. The router connecting to the public Internet will then share its single routable IP address with the devices on the home or private network.

In my home, the router, synology disk station (media server), and network printer get static (fixed) IP addresses. Everything else is dynamically assigned IP when they are powered on. Usually one picks a range of IP addresses for dynamic IP assignment, and the static IP addresses are outside of the dynamic range.

VikingOSX wrote:

In my home, the router, synology disk station (media server), and network printer get static (fixed) IP addresses. Everything else is dynamically assigned IP when they are powered on. Usually one picks a range of IP addresses for dynamic IP assignment, and the static IP addresses are outside of the dynamic range.

FWIW and given this is usually asked in the context of OS X Server, OS X Server itself needs a static IP address.

That'll typically be a NAT'd private static IP address in most network environments, with either a dynamic or a static public IP address acquired from the ISP, and with port forwarding configured at the gateway-firewall device if remote access into the server is required.