Section 29. Configure Networking Automatically with DHCP or BootP

29. Configure Networking Automatically with DHCP or BootP

BEFORE YOU BEGIN

27 Set Your Network Device Preference Order

SEE ALSO

30 Configure Networking Manually

40 Connect to the Internet Wirelessly

If you connect to the Internet using Ethernet or AirPort, chances are that your Mac is part of a business or home network that is configured to automatically set each computer's TCP/IP settings as soon as it connects. If this is the case, you don't have to do any difficult configuration to get online; you just have to set up your Mac to use the Dynamic Host Configuration Protocol (DHCP) the most popular protocol whereby the computer is automatically assigned an IP address and routing information by a server somewhere on the LAN.

29. Configure Networking Automatically with DHCP or BootP

KEY TERM

Dynamic Host Configuration Protocol (DHCP) A mechanism whereby you can plug a computer into a network and have it automatically receive TCP/IP configuration settings from a server somewhere on the network, freeing you from having to type in those settings yourself.

In DHCP (BootP, another similar protocol, is sometimes used instead), as soon as you plug in an Ethernet cable, join an AirPort network, or power-on a computer that's already plugged in, the computer sends out a broadcast message asking for a computer on the network to tell it what to use as an IP address , as well as other critical TCP/IP information, such as the subnet mask , the router , the DNS server , and the search domain . If a DHCP server is present on the LAN, it sends a reply directly back to your computer with the TCP/IP information it needs. Your computer applies that information to its own configuration, and it can then communicate on the network and with the Internet.

In the IPv4 protocol, your IP address uniquely identifies your computer on the Internet. This address is a string of four numbers from 0 to 255, separated by dots (periods). Another string of four numbers , the subnet mask, defines how large your network's address space is. Common subnet masks are 255.0.0.0 , 255.255.0.0 , and 255.255.255.0 ; they correspond to networks with 16.7 million, 65 thousand, and 256 IP addresses, respectively. Finally, the router, a device on your local network that enables your computer to communicate with computers on far-flung remote networks around the world, must be present within the address space of the local network defined by the combination of your IP address and your subnet mask.

A further bit of magic is done by DNS servers, which provide a mapping between numeric IP addresses and textual hostnames; this mapping is the Domain Name Service, or DNS. When you connect to the hostname www.apple.com , a DNS server must supply the IP address associated with that hostname before your computer can connect to it. If you specify a search domain, the hostnames you type can be shortened ; for instance, if your search domain were apple.com , you could simply type info to connect to info.apple.com . Generally you would use this option on a business network, and set it to your company's domain name so that you can connect to internal servers by using only their machine names .

KEY TERMS

IP address A unique numeric address that identifies your computer on the Internet. An IP address is of the form A.B.C.D , where each letter is any number from 0 to 255for example, 17.112.152.32 .

Subnet mask Another set of four numbers, each from 0 to 255, the subnet mask is a numeric string that defines how large the address space is on your local network.

Router A device on your local network that enables your computer to communicate with computers on other networks across the Internet.

DNS servers Computers on the network that provide a mapping between numeric IP addresses and textual hostnames; this mapping is the Domain Name Service, or DNS.

Search domain Allows you to type shortened versions of hostnames on the Internet. For instance, if you worked at Apple, your search domain would be apple.com , and you could simply type info to connect to info.apple.com.

IPv4 The current and ubiquitous version of IP, the Internet Protocol, is 4. IPv6 is the next -generation version of IP, and you can configure your Mac to use IPv6 if you're on a network that uses it (click the Configure IPv6 button)but most networks don't use IPv6 yet.

1.	Select Your Network Device On the Network Preferences pane of the System Preferences application, select Network Status from the Show menu. Double-click the network device you want to configure. Alternately, select the device from the Show drop-down menu. Make sure your network device is turned on and physically connected to the network. For instance, if you're configuring an AirPort card, make sure you're within range of the AirPort base station. If you're configuring an Ethernet connection, make sure the Ethernet cable is plugged in.
2.	Open TCP/IP Settings Click the TCP/IP tab to open the screen where you can configure the selected device's TCP/IP settings.
3.	Select DHCP or BootP From the Configure IPv4 drop-down menu, select Using DHCP or Using BootP , depending on what protocol your network uses. If you don't know which one to pick, select Using DHCP because that protocol is much more common.
4.	Apply TCP/IP Settings Click Apply Now to commit the configuration. The Mac sends out a DHCP request; in a few seconds, you should see the information fields become filled in with numeric data. You should now be able to connect to the network using your favorite applications. NOTE If the IP address that the Mac reports has a subtitle of Self-assigned , it means that it got no response to its DHCP or BootP request, and therefore wasn't able to obtain a valid configuration. This could be because the DHCP server is not working properly, or it could mean that your Mac is not connected properly to the network. Check your cabling; if it looks correct, contact your network administrator for assistance, or consult the documentation for the device providing DHCP service (such as your AirPort Base Station).
5.	Enter DNS Information and IP Address Normally, you don't have to enter any additional TCP/IP information. However, there are some circumstances under which you might need to enter an IP address or additional DNS information. For instance, you might have a computer that must use a fixed and predetermined IP address, and you must be assured that that IP address will never change, but you still want to be able to get other information, such as the subnet mask and router, automatically through DHCP. If your DHCP server does not provide DNS information, enter the IP addresses of the network's DNS servers into the DNS Servers boxenter the addresses one per line or separate them with commas. (Ask your network administrator or your Internet service provider for the addresses.) If you want, you can enter your company's domain name in the Search Domains field to access local servers more quickly. If you must specify a fixed IP address, use the Using DHCP with manual address option in the Configure IPv4 drop-down menu.