SEM 1V2 Chapter 10 Routing IP Addressing

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SEM 1V2 Chapter 10Routing IP Addressing
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Layer 3 - Routing and Addressing
10.1. Understand
Why It Is Necessary to Have a Network Layer.
10.2. Understand Path Determination.
10.3. Understand the Purpose and Operation
of IP Addresses within the IP Header.
10.4. Understanding and Working with IP Address
Classes. 10.5. Understand the Purpose
of Reserved Address Space. 10.6.
Understand the Basics of Subnetting.
10.7. Understand How to Create a Subnet.
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10.1. Understand Why It Is Necessary to Have a
Network Layer 10.1.1. Explain why
identifiers (names) are not enough for full
connectivity.. 10.1.2. Identify the
needs for multiple networkssegmentation and
autonomous systems. 10.1.3. Explain
why there is a need for communication between
separate networks.. 10.1.4.
Illustrate network-to-network connection Layer 3
devices and other devices..
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The purpose of this target indicator is to
justify the necessity of Layer 3 addresses. The
key distinction to make for the students is that
MAC addresses represent a flat address space.
That is, they are non-hierarchical like social
security numbers. MAC addressing -- the naming of
computers with hexadecimal numbers -- works fine
in a LAN environment, but they don't scale well.
As the number of computers and separate networks
grows, the necessity of some kind of hierarchical
addressing scheme becomes apparent. Telephone
and Postal codes are routing codes which are
analogous to layer 3 addressing schemes. As an
activity you might have the students drawing a
diagram for n 30 computers might help. Label
them A, B, C, etc. and then relabel and
reorganize the computers hierarchically with
two-part numerical codes. Discuss the
implications of both addressing schemes.
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There are two main points to this target
indicator. First, that multiple networks are
desirable (we create them to segment our networks
into smaller networks for traffic management) and
that multiple networks already exist (the
Internet is a WAN comprised of millions of
smaller networks all of which want to be somewhat
connected). Secondly, this target indicator makes
use of the highway analogy for networking. This
analogy was introduced in Chapter 1 and is a rich
analogy for many aspects of networking.
Particularly important is that routing takes
place in highway systems (perhaps have the
students brainstorm how this occurs -- i.e.,
maps, traffic signs and signals, people getting
directions, etc) and that large data networks
need routing information as well.
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The importance of this target indicator can be
rephrased as "why would we want to have an
Internet." The world is just beginning to answer
this question every day some new purpose is
found for the world-wide interconnection of
networks known as the Internet. The knowledge
sharing, the commerce, the near instantaneous
personalized communications, and many other
reasons are why separate networks would "need" to
communicate.
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There are three key points to this target
indicator routers connect separate networks,
routers make best path decisions based on layer 3
information, and routers actually switch packets
from incoming ports to appropriate outgoing
ports. You cannot stress these three points
enough -- everything that follows in Chapters 10
and 11 is in some way justified so the router can
perform one of these functions. Without routers
you could not connect separate networks
efficiently, there would be no devices
intelligent enough to route packets along a best
path nor to switch them to that best path.
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Pose the problem to the students -- how do we get
from point A to point B in a city at rush hour
when there's been an accident on the main
highway? This will illustrate the notion of best
path selection. Having a map of the city and
having the students choose best paths is a simple
and illustrative activity. You can then compare
this to routing processes. Having students
discuss best paths simulates routing protocols
(about which they will learn later). Again, the
idea is to make as many of the abstractions as
tangible as possible.
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10.2.2.1. Describe addressing as a function of
the network layer and as a way to achieve routing
We have made the distinction between "naming" a
computer with a MAC address and "addressing" a
computer with a network layer address. This
target indicator strives to emphasize the
difference. You might pose the problem to the
students -- would routing be possible if we just
had names (MAC addresses) for computers? What
problems would arise and what would such Layer 2
"routing" devices have to look like (amongst
other problems they would have to remember the
name of every single device on all networks in
order to route any information, hence the layer 2
routing tables would be ridiculously large). Then
emphasize how hierarchical addressing, when
combined with naming, gives us efficient local
delivery but also efficient world-wide routing
and delivery of information.
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10.2.3.1. Explain the importance of the Layer 3
function that enables computer mobility The
purpose of this target indicator is to emphasize
another benefit of a two-tiered, hierarchical
addressing scheme computers can be moved and the
network can accommodate moves with a minimum of
disruption. Computers keep their name (their MAC
address) but can change their address (their
network layer address).
The MAC address can be compared to your name, and
the network address to your mailing address. If
you were to move to another town, your name would
remain unchanged, but your mailing address would
indicate your new home.
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10.2.4.1. Compare and contrast flat and
hierarchical addressing The purpose of this
target indicator is both summary and
introduction. Flat and hierarchical addressing
schemes have been extensively mentioned. So this
summarizes the main points of those target
indicators. But a grand introduction is made the
network layer addressing scheme, the layer 3
protocol to be used in the class -- Internet
Protocol, or IP -- is introduced. IP addressing
is one of the most important topics throughout
all four semesters of the curriculum and on the
CCNA exam.
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The Internet Protocol (IP) is the preferred
addressing implementation of a hierarchical
network addressing scheme. As information flows
down the OSI model, data are encapsulated at each
layer. At the network layer, packets are turned
into datagrams, and if the network is using IP,
then IP datagrams are formed. IP determines the
form of the IP header, but does not concern
itself with the data.
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The purpose of this target indicator is that the
student be able to explain, in detail, what
comprises the IP datagram. Relate this datagram
-- a layer 3 PDU -- to the frame format diagrams
that students studied when learning about layer
2. This will make the concepts of headers and
fields more plausible. Have the students pay
particular attention to the source and
destination IP addresses. Also point out that
while the IP datagram looks complicated, all of
this "overhead" information is necessary for
routing and "best effort delivery" of packets.
Also note that the total length in bytes of this
"overhead" is typically a small fraction of the
total length of the entire packet -- it is mostly
carrying upper layer encapsulated data.
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10.3.3.1. Identify the source and destination
fields in an IP header, and explain their
purposes. The purpose of this target indicator
is to focus on the source and destination fields
of the IP datagram. Their length in IP version 4
is 32 bits this concept is introduced and the
fact that these addresses are necessary for
routing is emphasized.
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10.3.4.1. Define an IP address as a 32-bit
binary number. The purpose of this target
indicator is to show the binary format of an IP
address. Draw upon the binary math that was
taught in Chapter 1. Spend enough time on this
diagram to assure that all students have mastered
it all future work involving IP addressing
presupposes a complete understanding of the
binary format and powers of two involved.
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10.3.5.1. Identify the component fields of an
IP address. This target indicator introduces
two important IP addressing concepts dotted
decimal notation, and the classification of parts
of the address as "network" numbers and parts of
the address as "host" numbers. Relate the network
numbers to the earlier discussion of hierarchical
addressing, including the analogy to zip codes.
Practicing binary to decimal and decimal to
binary conversions would be appropriate here,
using the dotted decimal notation. Practice
Problems 1.Convert 1101 0101.1100
0011.00001111.0101 0101 to dotted decimal
notation. 2.Convert 156.1.149.9 to binary
notation.
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Students should be able to classify IP addresses
as A, B, or C. They should also be able to label
the octets "network" and "host" as appropriate
for that address class. Emphasize that the
network numbers are assigned by an external
agency only the host numbers can be assigned
locally. While you may have heard of other
class-less IP addressing schemes, the concepts of
A, B, and C addresses are still widely used. And
many questions on the CCNA exam assume classful
addressing.
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The InterNIC reserves Class A addresses for
governments throughout the world, Class B
addresses for medium-size companies, and Class C
addresses for every one else. When written in a
binary format, the first bit of a Class A address
is always 0. The first 2 bits of a Class B
address are always 10, and the first 3 bits of a
Class C address are always 101.
Class A 0 --127. 16,777,214
Class C 192-223 254
Class B 128-191 65,534
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Dotted decimal notation is for humans bits are
for computers!
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Use of calculators is discouraged for two
reasons. First, practitioners of networking often
need to make quick, "back-of-the-envelope"
conversions between decimal and binary numbers.
Second, no calculators are allowed on the CCNA
exam.
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10.4.4.1. Convert decimal IP addresses to their
binary equivalents.
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To convert binary IP addresses to decimal
numbers, use the opposite approach you used to
convert decimal numbers to binary numbers.
ExampleConvert the binary IP address
10101010.11111111.00000000.11001101 to a decimal
number. 1 0 1 0 1 0 1 0 27 26 25 24
23 22 21 20 128 0 32 0 8 0 2 0 128 32
8 2 170
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The purpose of this target indicator is to
introduce the concept of specially reserved IP
addresses. Have the students work out the basic
network numbers for all three classes of IP
address. For example, for a class A address
99.0.0.0 would be a reserved network number and
99.255.255.255 would be a broadcast number. For a
class B address 156.1.0.0 would be a reserved
network "wire" number and 156.1.255.255 would be
a broadcast number. For a class C address
203.1.17.0 would be a reserved network number and
203.1.17.255 would be a broadcast number. Also
be forewarned that once subnetworks are created,
the reserved network numbers and broadcast
numbers become less obvious and require more work
to compute.
10.5.1.1. Explain the existence of network ID and
broadcast address
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10.5.2.1. Identify network ID Instructor
Note The importance of this target indicator is
identifying the importance of network id numbers.
Two hosts with differing network id numbers
require a device, typically a router, in order to
communicate.
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Zip Codes and networks IDs are quite similar in
how they work. A network ID enables a router to
put a packet into the appropriate network
segment. From there, the host ID helps the
devices on the network determine the packet's
destination. Zip Codes enable the postal system
to direct your mail to your local post office,
and to your neighborhood. From there, the street
address directs the carrier to the proper
destination.
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10.5.4.1. Identify broadcast address. Compare and
contrast broadcast addresses to bulk mailings
The purpose of this target indicator is to use
the postal analogy for networking. Both the
postal system and internetworks use a form of
"collective" addressing. In the postal system, a
bulk mailing goes to everyone with a particular
postal code (typically a geographical region). In
internetworks, a broadcast goes to every host
with a particular network id number (typically a
region of a logical network topology). A
broadcast address is an address that has all 1s
in the host field. When a packet starts out on a
network that is using a broadcast address, all
devices on the network take notice of it.
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Students must recognize the number of bits in the
network and host portions of all three classes of
IP addresses. If they can recognize this, then it
is a matter of powers of two to determine how
many hosts are intrinsically part of classful IP
addressing.
Remember that the first address on each segment
is reserved for the network number and the final
address on each segment is reserved for
broadcasts.
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Network administrators sometimes need to divide
networks, particularly large networks, into
smaller networks, called subnetworks, in order to
provide extra flexibility. Most of the time
subnetworks are simply referred to as subnets
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To create a subnet address, a network
administrator borrows bits from the host field
and designates them as the subnet field. The
minimum number of bits that can be borrowed is 2.
If you were to borrow only 1 bit, to create a
subnet, then you would only have a network number
- the .0 network - and the broadcast number - the
.1 network. The maximum number of bits that can
be borrowed can be can any number that leaves at
least 2 bits, remaining, for the host number.
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The primary reason for using a subnet is to
reduce the size of a broadcast domain. Broadcasts
are sent to all hosts on a network or subnetwork.
When broadcast traffic begins to consume too much
of the available bandwidth, network
administrators may chose to reduce the size of
the broadcast domain.
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The longer name for subnet mask is instructive --
"extended network prefix". The mask's ones show
how far we are extending the network number (at
the expense of the host numbers).
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There are three fundamental operations in Boolean
algebra. These three functions are crucial in the
design of all digital circuits, and important in
programming. These functions are often used in
"Boolean searches" to use Internet search engines
to narrow the range of hits for a search. In
internetworking, the AND function is particularly
important part of the routing process. Teach
Boolean AND as similar to multiplication Boolean
OR as similar to addition and Boolean NOT as
simply inversion of the bit. It's also a good
time to review the different ways 1s and 0s are
sometimes represented -- ones as TRUE, ON, SHORT
CIRCUIT, 5 Volts and zeros as FALSE, OFF, OPEN
Circuit, or 0 Volts.
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The router performs the Boolean operations, the
most important of which is the AND operation. In
order to find the network ID of a subnet, the
router must take the IP address, and the subnet
mask, and logically, AND them together. The
resulting number is the network/subnetwork
number.
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10.7.1.1. Identify the range of bits that can be
borrowed to create subnets The purpose of this
target indicator is to correctly discern how many
bits may be "stolen" or "borrowed" from the host
fields to extend the network number. The first
step in this process is identifying the IP
address as class A (thus a default subnet mask of
255.0.0.0), class B (thus a default subnet mask
of 255.255.0.0), or class C (thus a default
subnet mask of 255.255.255.0). This establishes
the "minimum" mask. The maximum mask must leave
at least 2 bits for numbering hosts.
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10.7.3.1. Compute the number of subnets when you
are given a subnet mask and IP address There
are several techniques for calculating the number
of subnets when given the subnet mask and IP
address. From the IP address, you can determine
its class and hence the default subnet mask. Find
how many bits beyond the default mask the actual
subnet mask has been extended. This is the number
of bits "borrowed" or stolen to create
subnetworks. The formula 2n - 2, where n is the
number of bits stolen, gives the number of USABLE
subnetworks created. Another way to see this is
to write out the powers of two, and find the
exponent of two that matches the number of bits
stolen. Whatever that power of two equals (less
the 2 reserved numbers) gives the number of
subnets.
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10.7.4.1. Compute the number of hosts per
subnetwork when you are given a subnet mask and
IP address There are several techniques for
calculating the number of subnets when given the
subnet mask and IP address. From the IP address,
you can determine its class and hence the default
subnet mask. Find how many bits beyond the
default mask the actual subnet mask has been
extended. This is the number of bits "borrowed"
or stolen to create subnetworks. The formula 2m
- 2, where m is the number of bits NOT stolen,
gives the number of USABLE host numbers created.
Another way to see this is to write out the
powers of two, and find the exponent of two that
matches the number of bits NOT stolen. Whatever
that power of two equals (less the 2 reserved
numbers) gives the number of hosts per
subnetwork.
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10.7.5.1. Perform a Boolean AND operation to
compute a network number The key to
understanding the result of the Boolean ANDing of
an IP address and a subnet mask is to realize
that once created, subnetworks are valid network
numbers as far as the "outside" world is
concerned. So as with the earlier calculation,
the bit-by-bit ANDing of the IP address and the
subnet mask gives the subnetwork number.
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Types of IP Addressing Problems Problem Given
195.137.92.0 and needing 8 usable subnets, find
the subnetwork numbers, the ranges of host
numbers, and subnetwork broadcast numbers.
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Solution IP Address is a class C. Default
subnet mask is 255.255.255.0. We need to extend
the network number by enough bits to give 8
usable subnets. Stealing 2 bits yields 2 usable
subnets, stealing 3 bits yields 6 usable subnets,
so we must steal 4 bits to get 14 usable subnets,
of which we needed 8. This makes the subnet mask
255.255.255.240. So the Network number is
195.137.92.NNNN HHHH where Ns stand for network
extension bits (subnets) and Hs stand for host
numbers. Next we must number the subnets there
are 16 combinations of 4 bit binary numbers but
they retain their place value within the last
octet.
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An unfortunate by-product of creating subnetworks
is that the reserved network and broadcast
numbers now exist for each and every subnetwork
created. Thus entire blocks of IP addresses,
which begin with these subnetwork id and
subnetwork broadcast numbers, are wasted. So the
network administrator must strike a balance
between the number of subnets required, the hosts
per subnet that is acceptable, and the resulting
waste of addresses.
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There are certain IP address ranges reserved for
private IP addressing schemes. Not everyone needs
connectivity to the Internet. Another relevant
discussion is IP address depletion. Various
schemes are being pursued to deal with IP address
depletion. First there is NAT. Second there is
CIDR. Third there is IP v6. While all of these
have there benefits, students should be
well-grounded in classful IP addresses.
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The End of Chapter 10