host on the network there must be a BOOTP profile
with an IP address assignment in it. No two profiles can have
the same IP address. Those profiles might be used at the same
time and that would mean that two hosts have the same IP
address. A device uses BOOTP to obtain an IP address when
starting up. BOOTP uses UDP to carry messages. The UDP message
is encapsulated in an IP packet. A computer uses BOOTP to send
a broadcast IP packet using a destination IP address of all 1s,
255.255.255.255 in dotted decimal notation. A BOOTP server
receives the broadcast and then sends back a broadcast. The
client receives a frame and checks the MAC address. If the
client finds its own MAC address in the destination address
field and a broadcast in the IP destination field, it takes and
stores the IP address and other information supplied in the
BOOTP reply message. A step-by-step description of the process
is shown in Figures through . Web Links BOOTP
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sDefinition/0,290660,sid7_gci213818,00.html
Content
9.3 Obtaining an IP address
9.3.5 DHCP IP address management Dynamic host
configuration protocol (DHCP) is the successor to BOOTP. Unlike
BOOTP, DHCP allows a host to obtain an IP address dynamically
without the network administrator having to set up an
individual profile for each device. All that is required when
using DHCP is a defined range of IP addresses on a DHCP server.
As hosts come online, they contact the DHCP server and request
an address. The DHCP server chooses an address and leases it to
that host. With DHCP, the entire network configuration of a
computer can be obtained in one message. This includes all of
the data supplied by the BOOTP message, plus a leased IP
address and a subnet mask. The major advantage that DHCP has
over BOOTP is that it allows users to be mobile. This mobility
allows the users to freely change network connections from
location to location. It is no longer required to keep a fixed
profile for every device attached to the network as was
required with the BOOTP system. The importance to this DHCP
advancement is its ability to lease an IP address to a device
and then reclaim that IP address for another user after the
first user releases it. This means that DHCP offers a one to
many ratio of IP addresses and that an address is available to
anyone who connects to the network. Lab Activity Lab
Exercise: DHCP Client Setup This lab is to introduce Dynamic
Host Configuration Protocol (DHCP) and the process for setting
up a network computer as a DHCP client to use DHCP services.
Web Links Dynamic Host Configuration Protocol
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gci213894,00.html
Content 9.3
Obtaining an IP address 9.3.6 Problems in
address resolution One of the major problems in networking
is how to communicate with other network devices. In TCP/IP
communications, a datagram on a local-area network must contain
both a destination MAC address and a destination IP address.
These addresses must be correct and match the destination MAC
and IP addresses of the host device. If it does not match, the
datagram will be discarded by the destination host.
Communications within a LAN segment require two addresses.
There needs to be a way to automatically map IP to MAC
addresses. It would be too time consuming for the user to
create the maps manually. The TCP/IP suite has a protocol,
called Address Resolution Protocol (ARP), which can
automatically obtain MAC addresses for local transmission.
Different issues are raised when data is sent outside of the
local area network. Communications between two LAN segments
have an additional task. Both the IP and MAC addresses are
needed for both the destination host and the intermediate
routing device. TCP/IP has a variation on ARP called Proxy ARP
that will provide the MAC address of an intermediate device for
transmission outside the LAN to another network segment.
Content 9.3 Obtaining an IP
address 9.3.7 Address Resolution Protocol
(ARP) With TCP/IP networking, a data packet must contain
both a destination MAC address and a destination IP address. If
the packet is missing either one, the data will not pass from
Layer 3 to the upper layers. In this way, MAC addresses and IP
addresses act as checks and balances for each other. After
devices determine the IP addresses of the destination devices,
they can add the destination MAC addresses to the data packets.
Some devices will keep tables that contain MAC addresses and IP
addresses of other devices that are connected to the same LAN.
These are called Address Resolution Protocol (ARP) tables. ARP
tables are stored in RAM memory, where the cached information
is maintained automatically on each of the devices. It is very
unusual for a user to have to make an ARP table entry manually.
Each device on a network maintains its own ARP table. When a
network device wants to send data across the network, it uses
information provided by the ARP table. When a source determines
the IP address for a destination, it then consults the ARP
table in order to locate the MAC address for the destination.
If the source locates an entry in its table, destination IP
address to destination MAC address, it will associate the IP
address to the MAC address and then uses it to encapsulate the
data. The data packet is then sent out over the networking
media to be picked up by the destination device. There are two
ways that devices can gather MAC addresses that they need to
add to the encapsulated data. One way is to monitor the traffic
that occurs on the local network segment. All stations on an
Ethernet network will analyze all traffic to determine if the
data is for them. Part of this process is to record the source
IP and MAC address of the datagram to an ARP table. So as data
is transmitted on the network, the address pairs populate the
ARP table. Another way to get an address pair for data
transmission is to broadcast an ARP request. The computer that
requires an IP and MAC address pair broadcasts an ARP request.
All the other devices on the local area network analyze this
request. If one of the local devices matches the IP address of
the request, it sends back an ARP reply that contains its
IP-MAC pair. If the IP address is for the local area network
and the computer does not exist or is turned off, there is no
response to the ARP request. In this situation, the source
device reports an error. If the request is for a different IP
network, there is another process that can be used. Routers do
not forward broadcast packets. If the feature is turned on, a
router performs a proxy ARP. Proxy ARP is a variation of the
ARP protocol. In this variation, a router sends an ARP response
with the MAC address of the interface on which the request was
received, to the requesting host. The router responds with the
MAC addresses for those requests in which the IP address is not
in the range of addresses of the local subnet. Another method
to send data to the address of a device that is on another
network segment is to set up a default gateway. The default
gateway is a host option where the IP address of the router
interface is stored in the network configuration of the host.
The source host compares the destination IP address and its own
IP address to determine if the two IP addresses are located on
the same segment. If the receiving host is not on the same
segment, the source host sends the data using the actual IP
address of the destination and the MAC address of the router.
The MAC address for the router was learned from the ARP table
by using the IP address of that router. If the default gateway
on the host or the proxy ARP feature on the router is not
configured, no traffic can leave the local area network. One or
the other is required to have a connection outside of the local
area network. Lab Activity Lab Exercise: Workstation
ARP This lab is to to introduce Address Resolution Protocol
(ARP) and the arp –a workstation command.