1) DHCP automatically assigns IP addresses to devices on a network when they connect. A DHCP client broadcasts a request that is received by the DHCP server, which then issues an IP address from its scope. 2) The DHCP server sends an acknowledgement to the client with the assigned IP and MAC address stored in its database. This assignment is called a DHCP lease that typically lasts 5-8 days. 3) If a DHCP client cannot connect to the DHCP server, it will automatically generate an APIPA address in the 169.254/16 range, allowing it to still communicate on the local network but not access the internet without a default gateway from the DHCP server.

1. Data Communication
Part 7
Alex Fernandez

2. DYNAMIC IP ADDRESSING
◎ Dynamic IP addressing, better known as Dynamic
Host Configuration Protocol (DHCP)or the older
(and much less popular) Bootstrap Protocol
(BOOTP) , automatically as- signs an IP address
whenever a computer connects to the network.
DHCP (and BOOTP, but for simplicity I’ll just say
DHCP) works in a very simple process. First, a
computer is configured to use DHCP. Every OS
has some method to tell the computer to use
DHCP, like the Windows exampleshown in Figure

4. HOW DHCP WORKS
◎ Once a computer is configured to use DHCP, it’s called a
DHCP client. When a DHCP client boots up it automati-
cally sends out a special DHCP discovery packet using the
broadcast address. This DHCP discovery message asks:
◎ “Are there any DHCP servers out there?” (See Figure
7.39.)
◎ For DHCP to work, there must be one system on the LAN
running special DHCP server software. This server is
designed to respond to DHCP discovery requests with a
DHCP offer. The DHCP server is configured to pass out IP
addresses from a range (called a DHCP scope), a subnet

6. ◎ DHCP client sends out a DHCP request⎯a poor name
choice as it really is accepting the offer. The DHCP
server then sends a DHCP ac-knowledge and lists the
MAC address as well as the IP information given to
the DHCP client in a database(Figure 7.41).
◎ The acceptance from the DHCP client of the DHCP
server’s data is called a DHCP lease . A DHCP lease is
set for a fixed amount of time, usually 5 to 8 days. At
the end of the lease time, the DHCP client simply
makes another DHCP discovery message. The DHCP
server looks at the MAC address information and,
un-less another computer has taken the lease, will

7. LIVING WITH DHCP
◎ DHCP is very convenient and, as such, very popular. So popular that it’s very
rare to see a user’s computer on any network using static addressing.
◎ The single biggest issue is when a DHCP client tries to get a
◎ DHCP address and fails. It’s easy to tell when this happens because the
operating system will post some form of error telling you there’s a problem
(Figure 7.42) and the DHCP client will have a rather strange address in the
169.254/16 network ID.
◎ This special IP address is generated byAutomatic Private IP Ad-dressing
(APIPA). All DHCP clients are designed to generate an APIPA address
automatically if there’s no response to a DHCP dis-covery message. The client
generates the last two octets of an APIPA address automatically. This will at
least allow all the DHCP clients on a sin -gle network to continue to
communicate with each other because they are on the same network ID.
Unfortunately, there’s no way for APIPA to give a default gateway, so you’ll
never get on the Internet using APIPA. That pro -vides a huge clue to a DHCP
problem: you can communicate with other computers on your network, but

8. ◎ If you can’t get out to the Internet, use whatever tool your OS provides to check your IP address. If it’s an APIPA address,
you instantly know you have a DHCP problem. First of all, try to reestablish the lease manually. Every OS has some way to
do this. In Windows, you can type the following command:
◎ ipconfig /renew
◎ On a Macintosh, you can go to System Preferences and use the Network utility (Figure 7.43).
◎ Sometimes you might find yourself in a situation where your computer gets confused and won’t grab an IP address no
matter what you try. In these cases you should first force the computer to release its lease. In Windows, get to a command
prompt and type these two commands, each followed by pressing ENTER:
ipconfig /release
ipconfig /renew
◎ In UNIX/Linux and even Macintosh you can use the IFCONFIG command to release and renew your DHCP address. Here’s
the syntax to reease:
sudo ifconfig eth0 down
And here is the syntax to renew:

10. SPECIAL IP ADDRESSES
◎ The folks who invented TCP/IP created a number of special IP addresses you
need to know about. The first special address is 127.0.0.1, the loopback
address. When you tell a device to send data to 127.0.0.1, you’re telling that
device to send the packets to itself. The loopback address has a number of
uses. One of the most common is to use it with the PING command. We use
the command PING 127.0.0.1 to test a NIC’s capability to send and receive
packets.
◎ For the moment, however, let’s just look at the
◎ ranges of addresses that are designated private IP addresses:
◎ ■ 10.0.0.0 through 10.255.255.255 (1 Class A license)
◎ ■ 172.16.0.0 through 172.31.255.255 (16 Class B licenses)
◎ ■ 192.168.0.0 through 192.168.255.255 (256 Class C licenses)