TMO Quick Tip - Deciphering Network IP Addresses

by , 7:30 AM EDT, June 14th, 2006

Just because you connect your Mac to a network with an Ethernet cable or through AirPort, that doesn't mean everything will work as expected. Even if your Mac tells you that it has a network IP address, you may still have problems seeing other computers, surfing the Web, or checking your email. If you know what the parts of an IP address mean, however, you can start tracking down where the problem might be.

TCP/IP addresses are groups of unique numbers that identify your Mac, or any other device, on your network. TCP/IP address numbers always consist of four sets of numbers separated by periods like this: 216.22.45.40. There are three types of TCP/IP address numbers you are likely to encounter: private addresses, public addresses, and self assigned addresses.

Private Addresses
A private, or internal, TCP/IP address is a number that is assigned to identify your Mac on your local network. Addresses that appear on your local, private network typically start with either 192.168, 172.16^*, or 10.0 - 192.168.0.110 and 10.0.0.8 are both examples of private address numbers.

The first three sets of numbers should be the same for every device - both computers and networked printers - on your network. The last number is the unique identifier assigned to your Mac. For example, my current IP address is 192.168.0.110. Another Mac on the same network has been assigned 192.168.0.122.

Public Addresses A public, or external, TCP/IP address is typically assigned to a computer that sits outside of your private network. These addresses start with numbers other than 192 or 10. 216.22.45.40 is a public IP address. Actually, it is the IP address assigned to www.macobserver.com.

Self Assigned Addresses
If your Mac isn't able to obtain a network address, it will create an address of its own that starts with 169.254. That's when you know for sure that you have a network problem.

Finding Your IP Address To figure out what IP address is currently assigned to your Mac, check the Network Preference Pane in System Preferences. Here's how:

• Choose Apple menu>System Preferences to launch System Preferences.
• Click the Network button.
• Click the Show pop-up menu and choose Network Status.

The Network Preference Pane lists your potential network connections. A green dot next to a network connection means it is active and has a good IP address. A yellow dot means the network connection is active, but doesn't have a good IP address. A red dot means that the network connection hasn't been configured yet.

If Network Status doesn't show your current IP address, you can check the network connection yourself:

• Make sure you are still in the Network Preference Pane.
• Choose the network connection you want to check. If you are connected to your network via Airport, choose Airport. If you are using an Ethernet cable, choose Built-in Ethernet.
• Click the TCP/IP tab.
• Look in the IP Address field for your TCP/IP address number.

Diagnosing the cause of a network problem, like finding you have a self assigned IP address, can be an involved process, and would turn today's Quick Tip from "quick" to "long and drawn out." We'll delve into some things you can do to hunt down network problems in another Quick Tip. For now, remember that an IP address that starts with 169.254 means your Mac isn't being assigned an IP address and you won't be able to get on the Internet.

^*Okay... To get all technical on you, the IP address range from 172.16.0.0 through 172.31.255.255 is defined as private.

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