In most situations, an internet service provider (ISP) assigns a single external IP address to a customer. That address can be dynamic, in that it changes based on the needs of the ISP, or static, meaning that it never changes once it’s assigned (static IP addresses are often available only for an extra fee).
Once inside your network, a router, either standalone like an AirPort Extreme or one built in to a cable or DSL modem, handles the IP addresses of the devices that connect to it.
In most consumer networks, IP addresses are assigned dynamically as needed, just as external IP addresses are assigned by an ISP, ensuring that every device has a unique address and allowing for addresses to be recycled and reused as devices come and go from the network.
This works just fine with almost no configuration required in nearly all consumer network setups. However, in some configurations, such as in the case of a home server, it’s beneficial for certain devices to have consistent, known, static IP addresses.
Once the basic network concepts are understood, setting up a static IP address for a device on your network is a fairly straightforward process. For our purposes, we’ll focus on assigning an IP address to a Mac using an Airport Extreme router, with the screenshots and descriptions referring to the most recent versions of OS X (10.7.4) and Airport Utility (6.0).
First, we need to locate the Mac’s “Hardware Address,” a unique identifier for the computer also known as MAC address (that’s “Media Access Control,” not related to Apple Macintosh products) or, depending on the connection type, an Ethernet ID or Airport ID.
Head to System Preferences > Network and select the interface that you use to connect your Mac to the network from the list on the left. In our example, it’s a Wi-Fi connection.
Next, click “Advanced” in the lower right corner, and then select the Hardware tab on the right. Your MAC address will be listed at the top of this window. Jot it down or highlight it and press Command-C to copy it.
Now, head over to Airport Utility. Apple recently changed AirPort Utility’s interface quite a bit, so if you’re using version 6.0 or higher, select your primary router from the network chart and press “Edit.”
Select the “Network” tab and make sure your “Router Mode” is set to DHCP Only or DHCP and NAT.
Note: If your router mode is set to “Off” or “Bridge Mode,” that means another device on your network, such as an integrated modem and router from your ISP, is handling DHCP within your network and the Airport Extreme is simply passing the connection along via wireless and Ethernet.
Next, we need to look at the “DCHP Range.” This is the range of addresses that are available to the router for assignment to network devices. When we assign an IP address to our Mac, we want to pick a number outside this range, so that the router won’t attempt to assign it to another device.
In our example here, our default range is 10.1.10.2 to 10.1.10.200. Let’s raise the minimum address up a bit to create room for our assigned IP addresses. To do that, simply change the last value of the first address to something higher than 2. In our case, we’ll raise it to 20. This allows us to use an value between 10.1.10.2 and 10.1.10.19 for our assigned IP address.
Next, under DHCP Reservations, click the “plus” button to create a new static IP assignment. Give the assignment a description, such as “Home Server” and make sure that “Reserve Address By” is set to “MAC Address.”
Now, get the MAC address we found earlier for the computer you wish to assign a static IP to and type or paste it into the “MAC Address” field. Pick a value outside of the DHCP range discussed above and enter it into the “IPv4 Address” field. In our example, we’ll use 10.1.10.10, a nice and easy address to remember. Press “Save” to save your changes and close the window.
Finally, press “Update” to save your changes to the router. The router will quickly restart and your Mac will now be assigned a static IP address that is reserved by the Airport Extreme.
Now you’re free to reliably point other computers and devices to your Mac’s IP address or forward ports for various network applications, confident that the Mac’s IP address will always be the same.
Of note, a Mac can also obtain a static IP address by configuring the Network settings directly on the Mac. To do this, go to System Preferences > Network > Wi-Fi (or Ethernet, depending on connection type) > Advanced > TCP/IP.
Change the “Configure IPv4” entry to “Using DHCP with manual address” and enter your own IP address that is within the router’s subnet (meaning that the first three sets of numbers are the same, in our case 10.1.10.X), but outside of the router’s DHCP range (discussed above).
This is a bit easier than the steps above and works great if your Mac never leaves your local network. If your Mac is a portable and occasionally connects to other networks, or if you reconfigure your router, you’ll have to manually adjust the Mac’s IP settings each time. When the IP address is assigned via the router as described above, however, you’ll enjoy the “best of both worlds:” a static IP address on your local network and DHCP flexibility on other networks.