Section 11.2. Configuring IP Addresses

11.2. Configuring IP Addresses

When IP was first used, every computer on a network had to be hand-configured with a static IP address and a netmask address before it could communicate with other machines. While it's nice to have a static IP address, it also meant that it was possible to run out of IP addresses assigned to a domain. This often forced sysadmins to restrict certain users from access to network services (file sharing, printing, even the Internet) by not assigning an IP address to a certain class of machines or users, with dedicated "network" computers in each group from which the network could be accessed. As you can imagine, that was a real pain.

Fortunately for today's users, most networks provide Dynamic Host Configuration Protocol (DHCP ) services. These services allow computers to be configured automatically with an IP address and netmask for that particular network. DHCP also helps conserve IP addresses by letting a potentially large number of computers share a smaller number of active IP addresses. DHCP services are provided by a separate server on the network or can be built into the hardware, known as a gateway router, providing access to the Internet via an ISP.

When you plug your computer into a DHCP-enabled network, your machine sends out a low-level request to the DHCP server for configuration information. The DHCP server responds by sending the IP address, netmask, router, and DNS information that the computer needs to connect to the network. Your computer uses this information to configure itself and becomes a fully functional node on the network.

DHCP works only when the network has been configured properly. In the absence of a DHCP server, Mac OS X uses a link local address, which is part of Bonjour. This is a private network address in the range 169.254.1.0 to 169.254.254.255. By assigning itself an address when no other address information is present, your Mac can still interact with other hosts on the network that also have self-assigned IP addresses. This is perfect for setting up ad hoc networkssuch as when you want to connect two laptops together with an Ethernet or FireWire cable. This address won't work for accessing the Internet, but it works for connecting a group of computers together to share data, compile code, and the like.

IP over FireWire (IPoFW) was introduced with Mac OS X Panther (Version 10.3). The benefit of IPoFW is it often offers higher data transfer rates (up to 400 MB per second for standard FireWire, or 800 MB per second between FireWire Extreme-equipped Macs) over traditional Ethernet connections (either 10 or 100 MB per second).

11.2.1. Manually Configuring IP Information

If you need to manually configure your IP information so your Mac can access a network or the Internet, the place to do so is in the Network preference panel, shown in Figure 11-4. You'll need the following pieces of information from your network administrator or ISP:

• The IP address assigned to your computer

• The subnet mask for the address

• The router, also called a gateway by many admins, that your computer should use to communicate with the Internet

• The DNS servers your computer should use

While it is possible to configure your IP information using ifconfig, it is not recommended. Settings made with ifconfig are not reflected in the Network preference panel and will be overridden by any changes made in the GUI. If you want to use a command-line tool, you should take a look at ncutil by Jeff Frey. You can find it online at http://deaddog.duch.udel.edu/~frey/darwin/ncutil/.

After entering the information for your network connection, you'll need to click the Apply Now button for the settings to take effect. Check your connection's status by going to the Show pop-up and select Network Status from the menu. If you see a green dot, your connection is working; a red dot, as mentioned earlier, means that your connection isn't working properly.