time this offered adequate scalability.
Unfortunately, the designers of TCP/IP could not have predicted
that their protocol would eventually sustain a global network
of information, commerce, and entertainment. Over twenty years
ago, IP Version 4 (IPv4) offered an addressing strategy that,
although scalable for a time, resulted in an inefficient
allocation of addresses. The Class A and B addresses make up 75
percent of the IPv4 address space, however fewer than 17,000
organizations can be assigned a Class A or B network number.
Class C network addresses are far more numerous than Class A
and Class B addresses, although they account for only 12.5
percent of the possible four billion IP addresses.
Unfortunately, Class C addresses are limited to 254 usable
hosts. This does not meet the needs of larger organizations
that cannot acquire a Class A or B address. Even if there were
more Class A, B, and C addresses, too many network addresses
would cause Internet routers to come to a stop under the burden
of the enormous size of routing tables required to store the
routes to reach each of the networks. As early as 1992, the
Internet Engineering Task Force (IETF) identified the
following two specific concerns: - Exhaustion of the
remaining, unassigned IPv4 network addresses. At the time, the
Class B space was on the verge of depletion.
- The
rapid and large increase in the size of Internet routing tables
occurred as more Class C networks came online. The resulting
flood of new network information threatened the ability of
Internet routers to cope effectively.
Over the past
two decades, numerous extensions to IPv4 have been developed.
These extensions are specifically designed to improve the
efficiency with which the 32-bit address space can be used. Two
of the more important of these are subnet masks and classless
interdomain routing (CIDR), which are discussed in more detail
in later lessons. Meanwhile, an even more extendible and
scalable version of IP, IP Version 6 (IPv6), has been defined
and developed. IPv6 uses 128 bits rather than the 32 bits
currently used in IPv4. IPv6 uses hexadecimal numbers to
represent the 128 bits. IPv6 provides 640 sextrillion
addresses. This version of IP should provide enough addresses
for future communication needs. Figure shows IPv4 addresses
which are 32 bits long, written in decimal form, and separated
by periods. IPv6 addresses are 128 bits long, written in
hexadecimal form, and separated by colons. IPv6 fields are 16
bits long. To make the addresses easier to read, leading zeros
can be omitted from each field. The field :0003: is written
:3:. IPv6 shorthand representation of the 128 bits uses eight
16-bit numbers, shown as four hexadecimal digits. After years
of planning and development, IPv6 is slowly being implemented
in select networks. Eventually, IPv6 may replace IPv4 as the
dominant Internet protocol. Web Links IPv4 vs. IPv6
http://www.comp.lancs.ac.uk/computing/ users/sschmid/Spie/
node5.html
Content 9.3 Obtaining an IP
address 9.3.1 Obtaining an Internet
address A network host needs to obtain a globally unique
address in order to function on the Internet. The physical or
MAC address that a host has is only locally significant,
identifying the host within the local area network. Since this
is a Layer 2 address, the router does not use it to forward
outside the LAN. IP addresses are the most commonly used
addresses for Internet communications. This protocol is a
hierarchical addressing scheme that allows individual addresses
to be associated together and treated as groups. These groups
of addresses allow efficient transfer of data across the
Internet. Network administrators use two methods to assign IP
addresses. These methods are static and dynamic. Later in this
lesson, static addressing and three variations of dynamic
addressing will be covered. Regardless of which addressing
scheme is chosen, no two interfaces can have the same IP
address. Two hosts that have the same IP address could create a
conflict that might cause both of the hosts involved not to
operate properly. As shown in Figure , the hosts have a
physical address by having a network interface card that allows
connection to the physical medium. Web Links Getting an
Internet Name and Address http://iishelp.web.cern.ch/IISHelp/
iis/htm/core/ iinmadd.htm
Content
9.3 Obtaining an IP address
9.3.2 Static assignment of an IP address Static
assignment works best on small, infrequently changing networks.
The system administrator manually assigns and tracks IP
addresses for each computer, printer, or server on the
intranet. Good recordkeeping is critical to prevent problems
which occur with duplicate IP addresses. This is possible only
when there are a small number of devices to track. Servers
should be assigned a static IP address so workstations and
other devices will always know how to access needed services.
Consider how difficult it would be to phone a business that
changed its phone number every day. Other devices that should
be assigned static IP addresses are network printers,
application servers, and routers. Web Links IP
Addresses http://www.microsoft.com/windows2000/
en/server/help/default.asp?url=/windows2000/
en/server/help/ip_addresses.htm
Content
9.3 Obtaining an IP address
9.3.3 RARP IP address assignment Reverse Address
Resolution Protocol (RARP) associates a known MAC addresses
with an IP addresses. This association allows network devices
to encapsulate data before sending the data out on the network.
A network device, such as a diskless workstation, might know
its MAC address but not its IP address. RARP allows the device
to make a request to learn its IP address. Devices using RARP
require that a RARP server be present on the network to answer
RARP requests. Consider an example where a source device wants
to send data to another device. In this example, the source
device knows its own MAC address but is unable to locate its
own IP address in the ARP table. The source device must include
both its MAC address and IP address in order for the
destination device to retrieve data, pass it to higher layers
of the OSI model, and respond to the originating device.
Therefore, the source initiates a process called a RARP
request. This request helps the source device detect its own IP
address. RARP requests are broadcast onto the LAN and are
responded to by the RARP server which is usually a router. RARP
uses the same packet format as ARP. However, in a RARP request,
the MAC headers and "operation code" are different
from an ARP request. The RARP packet format contains places for
MAC addresses of both the destination and source devices. The
source IP address field is empty. The broadcast goes to all
devices on the network. Therefore, the destination MAC address
will be set to all binary 1s. Workstations running RARP have
codes in ROM that direct them to start the RARP process. A
step-by-step layout of the RARP process is illustrated in
Figures through . Web Links Reverse Address Resolution
Protocol http://searchnetworking.techtarget.com/
sDefinition/0,,sid7_ gci214257,00.html
Content
9.3 Obtaining an IP address
9.3.4 BOOTP IP address assignment The bootstrap
protocol (BOOTP) operates in a client-server environment and
only requires a single packet exchange to obtain IP
information. However, unlike RARP, BOOTP packets can include
the IP address, as well as the address of a router, the address
of a server, and vendor-specific information. One problem with
BOOTP, however, is that it was not designed to provide dynamic
address assignment. With BOOTP, a network administrator creates
a configuration file that specifies the parameters for each
device. The administrator must add hosts and maintain the BOOTP
database. Even though the addresses are dynamically assigned,
there is still a one to one relationship between the number of
IP addresses and the number of hosts. This means that for every