Introduction to Computer Networking
Introduction to Computer NetworkingIP Addressing and
subnetting
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In This Session. . . .IP addressing Reserved addressesNetwork
identityHost identitySubnetworkingSubnet masks2
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Internet Layer - IP AddressesHierarchical addressing scheme32
bit addressDotted decimal notationUsed for addressing and
routingDivided into 2 main sectionsNetwork identityHost
identity3
www.cyberlabzone.comIP version 44 bytes divided into 4
sectionsEach section can range from 0 to 255If any portion of the
IP address contains a number greater than 255 it is ILEGAL
!Presented with dots in between 4 bytese.g.
193.60.61.24311000001.00111100.00111101.111100114
www.cyberlabzone.comFor subnetting use BINARYAlways work in
BinaryAll solutions can be discovered in BinaryEventually you will
see shortcutsUse Binary firstIt will take time to
understandPractice makes perfect5
www.cyberlabzone.comAddress Classes (a history lesson)Classes A,
B, C, D, E were createdIP numbers that began with : 1 to 126 are
class A addresses128 to 191 are class B addresses 192 to 223 are
class C addresses 224 to 239 are class D addresses 240 to 254 are
class E addresses.6
www.cyberlabzone.comExercise 1 class identification To which
class do the following IP numbers legally belong ?
193.45.67.12322.23.24.25131.74.59.12210.217.12,98193.260.50.177
www.cyberlabzone.com8Class C addressesFirst 3 bytes is network
portionLast byte for hosts on networkHow many hosts can class C
have ?C
www.cyberlabzone.comClass B addressesFirst 2 bytes is network
portionLast 2 bytes for hosts on networkHow many hosts can class B
have ?9
B
www.cyberlabzone.comClass A addressesFirst byte is network
portionlast 3 bytes for hosts on networkHow many hosts can class A
have ?
10A
www.cyberlabzone.comThe end of Address class systemClasses
wasted addresses through inflexibilityA better way of dividing
network space was soughtThis was known as Variable Length Subnet
Masking or VLSMNow we have to explicitly show the boundary of the
host and network portion of an address11
www.cyberlabzone.comCaveat to classless addressingMany people
and textbooks still refer to the address class systemNo networks
EVER use the class system in a production environmentBe aware of
the translations between the classful and classless systemRemember
ALWAYS to use the classless system despite the terminology of the
question12
www.cyberlabzone.comSubnet Mask identifies the boundarySometimes
called the Extended Network PrefixIdentifies the two sections of
the IP address, network and host fieldsUsed by routers to work out
the network a particular host belongs toWritten either as dotted
decimal or slash notation13
www.cyberlabzone.comSubnet Mask two notationsDotted decimal is
one of the notations255.255.255.0In binary this is
11111111.11111111.11111111.00000000Slash is the second
notationCount the number of 1sThere are 24 1sSo another way of
expressing this subnet mask is /2414
www.cyberlabzone.comNetwork (or wire) AddressThis is the
identity of the network and is used by routers to deliver packets
across networksNo host is ever given this addressTo discover the
network address, convert IP address from decimal to binaryReplace
the host section with 0sConvert back to decimal15
www.cyberlabzone.comFinding the Network Identity175.13.155.121
/1610101111.00001101. 10011011. 01111001What address class was this
?Which is the host field ?Replace with 0sConvert back to dotted
decimal
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www.cyberlabzone.comBroadcast Address RFC 919Each network needs
a broadcast addressNo host is ever given this addressA broadcast
can message all hosts in a LANUseful when a host needs to find
information without knowing exactly what other host can supply it
e.g. ARPWhen a host wants to provide information to a large set of
hosts in a timely manner
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www.cyberlabzone.comFinding the Broadcast NumberSubstitute the
host section of the binary version of the IP address with 1sConvert
back to dotted decimalWhat is the broadcast address for the IP
175.13.155.121 /16
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www.cyberlabzone.comReserved addressing numbersHost bits set to
0 are Network addressHost bits set to 1 are Broadcast address127
network numbers reserved for loopback127.0.0.1 is my NIC192.168.x.x
& 172.16-31.x.x & 10.x.x.x are non-routable private
addresses for NAT and private networks only defined in RFC
191819
www.cyberlabzone.comExercise 2Write the network & broadcast
number
12.123.14.235 /24177.177.177.177 /16220.17.124.14
/24277.14.13.86 /3320
www.cyberlabzone.comHost numbers on networksOriginal assignment
of classes wasted many IP addresses ( 3 bears problem )Class A for
governments with 224 hostsClass B can have 65 534 hostsmore than
most companies requireClass C can have 254 hostsless than most
companies require21
www.cyberlabzone.comSubnetworkingSolution is to subdivide
networksTake a network numberDivide it into smaller networksThese
are called subnetworks (subnets)
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www.cyberlabzone.comSubnettingTake a /24 (class C) address24
network bits & 8 host bitsCompany may need different
departmentsWe can borrow bits from host field to augment network
field
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www.cyberlabzone.comSubnetting /24 (Class C) addresse.g.
198.234.125.011000110.11101010.01111101.00000000Write network
number10111100.11101010.00000000.00000000Borrow 4 bits from host
field10111100.11101010.00000000.000000004 bits remain for host
identity30/09/201524
www.cyberlabzone.comSubnetted /24 (Class C)How many hosts does
this allow on each subnet ?How many subnetworks can we use ?First
and last subnet numbers are reservedFirst and last host numbers are
reserved25
www.cyberlabzone.comMore SubnettingCan borrow 2 host bits
minimumMaximum host bits that can be borrowed is all but the
rightmost 2 bits of the IP addressWhy is this so ?
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www.cyberlabzone.comCalculating subnet mask1. Express the
subnetwork IP address in binary form2. Replace the network and
subnet portion of the address with all 1s3. Replace the host
portion of the address with all 0s4. Now convert the binary
expression back to dotted-decimal notation27
www.cyberlabzone.comExample of subnet maskCalculate the subnet
mask for the Subnetted class C address 199.177.166.34 that has
borrowed 3 bits from the host field
11000111.10110001.10100110.0010001027 bits for network identity -
substitute these 27 bits for 1sChange remaining 5 host bits to
0sConvert to dotted decimal255.255.255.22428
www.cyberlabzone.comExercise 3Calculate subnet masks for class B
that has borrowed 4 bits from host field 155.233.2.13Calculate
subnet masks for class C that has borrowed 5 bits from host field
200.123.23.3
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www.cyberlabzone.comUse of Subnet MaskMask is ANDed with IP
address of hostThis gives network identityCan now be used to route
the message30
www.cyberlabzone.comExampleWhat is the network number for the IP
address 199.177.166.34 given that the subnet mask is
255.255.255.224Convert to binaryAND both numbers togetherResult is
network identity31
www.cyberlabzone.comExercise 3IP network address 213.72.83.0.
You require at least 17 subnets. Calculate1. The required subnet
mask2. The number of total subnets3. The number of available host
IP per subnet4. The first four subnet addresses5. The valid range
of host addresses for the first four subnets6. The Broadcast
address for each subnet32
www.cyberlabzone.comConclusion3 Classes available, A, B,
CStrictly not used any longer but well understoodFlexibility gained
by subnettingSubnets concealed from outside networks using subnet
masksMasks are ANDed with IP address of host to discover network
identity33
www.cyberlabzone.comHosts on class AThere are a possible 224 =
16 777 216 combinations available for the last 24 bitsSubtract the
two combinations that cannot be used as host addressesA class A
address can have 16 777 216 - 2 = 16 777 214 different hosts34
www.cyberlabzone.com35Hosts on class BThere are a possible 216 =
65 536 combinations available for the last 16 bitsSubtract the two
combinations that cannot be used as host addressesA class B address
can have 65 536 - 2 = 65 534 different hosts
www.cyberlabzone.com36Hosts on class CThere are a possible 28 =
256 combinations available for the last 8 bitsSubtract the two
combinations that cannot be used as host addressesA class B address
can have 256 - 2 = 254 different hosts
www.cyberlabzone.comNetwork Address10101111.00001101.
00000000.00000000175.13.0.0
It means this network 37
www.cyberlabzone.comBroadcast number10101111.00001101.
11111111.11111111175.13.255.255Reaches all hosts on a networkWill
not reach other hosts on different network
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www.cyberlabzone.comClasses & numbers12.123.14.0
12.123.14.255 (looks like Class A)177.177.0.0 177.177.255.255
(Class B)220.17.124.0 220.17.124.255 (Class C)277.14.13.86 Illegal
numberAnd illegal subnet mask
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www.cyberlabzone.comSubnetted Class C 24 = 16 -2 = 14 hosts
24 = 16 subnetworksNote that 14 hosts x 16 subnets is less than
256Why is this ?40
www.cyberlabzone.comSubnet
masks11111111.11111111.11110000.00000000255.255.240.0
11111111.11111111.11111111.11111000255.255.255.248
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www.cyberlabzone.comNetwork mask discoveryConvert to dotted
decimal199.177.166.32How many hosts on this subnet ?
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www.cyberlabzone.comExercise 3Need 17 subnets. Calculate bits
required for 17 subnets = 5Leaves 3 bits for host identity on
subnetsWhen calculating the first four subnet addresses, you will
begin with:00000000 = 000001000 = 800010000 = 1600011000 = 24
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www.cyberlabzone.comExercise 3 contd3 bits for the host leaves
23 - 2 = 6 host addresses per subnetValid host range213.72.83.1
213.72.83.2213.72.83.3 213.72.83.4213.72.83.5 213.72.83.644
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Thank You
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