Sub Netting and Routing

8/3/2019 Sub Netting and Routing

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SUBNET & ROUTINGSUBNET & ROUTINGSUBNET & ROUTINGSUBNET & ROUTING

Sritrusta SukaridhotoSritrusta Sukaridhoto

LecturerLecturerPens EEPISPens EEPIS--ITSITS


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Netmask / Subnet maskNetmask / Subnet masktutorialtutorial

Netmask / Subnet maskNetmask / Subnet masktutorialtutorial


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IP AddressingIP AddressingIP AddressingIP Addressing


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Subnet & Routing 4

IP addressing Introduction to IP addressing

Classes of IP addressing

Why Subnet Masks are necessary?

How to create subnet masks


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Why are IP addresses

written as bits? In order for data to pass along the media, it must

first be changed to electrical impulses.

When a computer receives these electricalimpulses, it recognizes two things: the presenceof voltage on the wire or the absence of voltageon the wire.


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What format do IP

addresses use?


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How are IP addresses

expressed in dottednotation?


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Why are IP addresses

necessary? IP addressing makes it possible for data

passing over the network media of the

Internet to find its destination. Because each IP address is a 32-bit value,that means that there are four billiondifferent IP address possibilities.

IP addresses are hierarchical addresseslike phone numbers and zip codes.


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How do IP addresses make itpossible for data sent via the

Internet to find its destination?

It is because each network connected to theInternet has a unique network number.

To ensure that each network number on theInternet will always be unique and unlike thatof any other number, an organization called

the International Network InformationCenter, or InterNIC


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How do IP addresses

incorporate network addresses? Every IP address has two parts. These are

known as the network number and the host

number.


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What are the different

classes of networks? There are three classes of IP

addresses that a company can receive

from the InterNIC. The InterNICreserves class "A" IP addresses forgovernments throughout the world,

class "B" IP addresses for mediumsize companies, and class "C"


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IP addressing Classes of Networks

N H H HClass A:

N N N HClass C:

N N H HClass B:

Network number assigned by NIC

Host number assigned by Systems Administrator

1 Byte 1 Byte 1 Byte 1 Byte


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IP addressing IP Address Bit Patterns

0 Network# Host #Class A:

Bit #1 2 - 8 9 32 (24 bits)

Class A address range

1.0.0.0 126.0.0.0 (127.0.0.0 is for loopback)

Private Class A address: 10.0.0.0

Number of hosts: 224 -2 = 16,777,214

8 16 24 32


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Class B:

Bit #1 3 - 16 17 32 (16 bits)

Class B address range

128.0.0.0 191.255.0.0

Private Class B : 172.16.0.0 172.31.0.0

Number of hosts: 216 - 2 = 65,534

1 0 Network # Host #

2

8 16 24 32


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Class C:

Bit #1 4 - 24 25 32 (8 bits)

Class C address range

192.0.0.0 233.255.255.0

Private Class C : 192.168.0.0

Number of hosts: 28 - 2 = 254

2

8 16 24 32

1 1 0 Network # Host #

3


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How many classes of

Networks are there? you have learned about three classes

of networks that can be assigned by

the InterNIC. In fact, there are five classes of

networks. However, only three of

these are used commercially.


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reserved for multicast

purposes and experimental

purposes? The highest number listed was 223.

You may have wondered why the

highest value was only 223 and not255, since there are 255 possiblevalues for an octet.

in IP addresses the values 224through 255 are not used in the firstoctet for networking purposes.


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What IP addresses are

reserved for the Networks? By convention, in IP addressing schemes,

any IP address that ends in all binary

zeroes is reserved for the networkaddress.

Thus, in a class "A" network, 113.0.0.0would be the IP address of that network.

Routers use a network's IP address whenforwarding data on the Internet.


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What IP addresses are

reserved for broadcasts? for the network that is 176.10.0.0, the broadcast

address that would be sent out to all devices onthat network would be 176.10.255.255.


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Who assigns subnet

addresses? As with the host number portion of

class "A," class "B," and class "C

addresses, subnet addresses areassigned locally.

Usually this is done by the network

administrator.


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IP addressing Subnet

mask Subnet Masking - Why?

131 181 115 252IP address

255 255 0 0Netmask

131 181 0 0

10000011 10110101 01110011 11111100

11111111 1111111100000000

00000000

0000000 0000000=

Network

Address

&


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IP addressing -

Subnetting Subnetting

Is the act of borrowing bits from the host

portion to create smaller networks (calledsubnetworks)

Minimum bits that can be borrowed is 2 -why?

Subnetting is used to reduce the number ofbroadcast domains

Communication between these subnetworks isachieved through a router


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How are subnet addressesconcealed from outside

networks? Subnets are hidden from outside

networks by using a mask.

These are referred to as subnetmasks.

The function of a subnet mask is to

tell devices which part of an addressis the network number including thesubnet, and which part is the host.


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What format do subnet

masks use? Subnet masks use the same format

as IP addressing.

In other words, they are thirty twobits long and divided into four octets.

Subnet masks have all 1s in the

network and subnetwork portion, andall 0s in the host portion.


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How many bits can be borrowed fromthe host number in class "B" and class

"C" networks to create subnets?

Because there are only two octets in thehost field of a class "B network, up to

fourteen bits can be borrowed to createsubnetworks.

A class "C" network has only one octet inthe host field. Therefore, only up to six

bits can be borrowed in class "C networksto create subnetworks.


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What happens to the subnet maskaddress if only some of the bits in an

octet are borrowed? Imagine that you have a class "B network. This

time however, instead of borrowing all eight bitsof the third octet, only seven bits are borrowed

to create subnetworks. Using binary representation, in this example, the

subnet mask would be11111111.11111111.11111110.00000000.

Therefore, 255.255.255.0 can no longer be usedas the subnet mask.


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If only seven bits are borrowed in a class"B" network, what would the subnet mask

be in dotted decimal notation? HINT: To convert any eight bit binary number

into a decimal number, total the powers of 2 that

occur in the number.


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What determines how many subnetworkscan be created by borrowing bits from

the host field?

Can you figure out all of the possiblecombinations of 0s and 1s if four bits are

borrowed from the host field to createsubnetworks?

16 from 0000 to 1111. However, you knowthat 1111 is reserved for broadcast and

0000 means this network.


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How many subnetworks can becreated by borrowing five bits

from the host field? Answer: Thirty-two

subnetworks or 25 =32

subnetworks can be created byborrowing five bits from thehost field.


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Which numbers in a subnetwork

are reserved for broadcasts? In previous section, we used an example of

a class "C" network in which three bits are

borrowed from the host field. You learnedthat when three bits are borrowed fromthe host octet, up to eight subnetworkscan be created each having up to thirty-

two hosts. You also learned that IP addresses endingin all binary 1s are reserved forbroadcasts. The same is true for

subnetworks.


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For IP address 197.15.22.160 on the sameclass "C network. How would this be

expressed in a binary numbering scheme? Answer: If 197.15.22.160 is converted to

binary format, it becomes

110001010.00001111.00010110. 10100000. The first three bits in the last octet, 101,indicate that this is the sixth subnetwork.As before the remaining bits are all binary

0s. This means that the IPaddress197.15.22.160 must be one that isreserved for a subnetwork address.


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Subnet mask How do we determine how many bits to

borrow for a subnet?

Determine the number of sub networksrequired

Work from the MOST significant (LHS)bits of the first octet after the network

number and calculate the number of bitsneeded to create the required number ofsubnetworks


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Subnet mask Example: You are given a class B address and you are

required to create 1000 subnetworks.

By working from the LHS of the (first octetafter the network number) 3rd octet,calculate the number of bits to equal or slightlyexceed 1000. (ie 2x = > 1000)

This would equate to 210 or 1024-2 networks

Hence you will need to borrow 10 bits from thehost portion to create 1000 subnetworks


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Subnet mask Example:

The subnetmask in this instance would

be255.255.255.192

How many host per network can youobtain from this addressing scheme?

11111111 11111111 11111111 11 000000

10 Bits Borrowed

(subnetmask)Natural Class B netmask6 bits left

for hosts


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Subnet mask How do we determine how many bits to

borrow for a subnet given the number ofhosts required?

Determine the number of hosts required Work from the LEAST significant (RHS)

bits of the last octet and calculate thenumber of bits needed to create the

required number of subnetworks


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Subnet mask Example: You are given a class B address and you require

1000 nodes per subnet

By working from the RHS (last octet) of the4th octet, calculate the number of bits toequal or slightly exceed 1000. (ie 2x = > 1000)

This would equate to 210 or 1024-2 networks Hence you will need to borrow 6 bits from the

host portion to create subnetworks with 1000hosts each


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Subnet mask Example:

The subnetmask in this instance wouldbe255.255.252.0

How many subnetworks per network canyou obtain from this addressingscheme?

Note: Do you recognise this address asthe student supernet address?

11111111 11111111 111111 00 00000000

6 Bits Borrowed

(subnetmask)Natural Class B netmask10 bits required

for hosts


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What about a Supernet? A supernet borrows bits from the networkportion to create contiguous nodes to form asuper network

For example Company A has about 1000 nodes to address. A class Baddress would be too big (or may not be available).Solution Supernetting using 4 contiguous class Caddresses

203.10.112.0203.10.113.0203.10.114.0203.10.115.0

(All netmasked to 255.255.255.0)


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What about a Supernet? By allowing the first 2 bits of the third octet

to be borrowed, a virtual class B address canbe created.

A supernet address of 203.10.112.0

203.10.115.255 is formed with a subnet maskof 255.255.252.0. The Host portion will be expanded from 8 10

bits

Route summarisation can occur to203.10.112.0/22


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What about a Supernet?

1100101

1

00001010011100

00

0000000

011001011

0000101001110001

00000000

11001011

0000101001110010

00000000

11001011

0000101001110011

11111111

11111111

1111111111111100

00000000

Host portionNetwork Portion

203.10.112.0

203.10.115.255

203.10.113.0

203.10.114.0

255.255.252.0

We have expanded the host portion by 2 bits to 10 bits


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What is a Broadcast

Address A broadcast address is used to by a

node to communicate with ALL nodes

in a broadcast domain Like the netmask, the broadcastaddress is AND with the networkaddress.

However, the host portion of thenetwork is identified in a broadcastaddress


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What is a Broadcast

Address A broadcast address does this by inserting all1s in the host portion.

Eg A natural class B broadcast address would look

something like this N.N.255.255 If it is not a classful subnetmask, you can

determine the broadcast address within eachsubnet by locating the host portion and setting

them to all 1s.


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What is a Broadcast

Address An example of a Broadcast address

131 181HostID

HostID

131 181 255 255

131 181HostID

HostID

&

=


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What is a Broadcast Address

An example of a cross boundary subnetbroadcast address with a mask of255.255.252.0

131 181 11[2,3,4,5] x

131 181 115 255

131 181 SN + HHostID

&

=

10000011

1011010101110000

00000000

10000011

10110101011100xx

xxxxxxxx

1000001 011100 1111111

Host portionNetwork Portion SNNetworkAddress

Broadcast

Address

Host


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What is a Broadcast Address

An example of a cross boundary subnetbroadcast address with a mask of255.255.252.0

In this example, IP addresses 131.181.112.0 131.181.115.255 belongto the same subnetwork

10000011

1011010101110000

00000000

10000011 10110101

01110001

00000000

10000011

1011010101110010

00000000

1000001

110110101

011100

11

1111111

1

Host portionNetwork Portion SN131.181.112.0

(Network)

131.181.115.255

(Broadacast)

131.181.113.0

131.181.114.0

k dd


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Network AddressVS

Broadcast address Remember

A Network address has all the hostbits set to 0

A Broadcast address has all the hostbits set to 1

Therefore 131.181.112.0 is the network address

131.181.115.255 is the broadcast address


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Network AddressVS

Broadcast address This is important when you are doing

ifconfig and routing commands

For example, if a host has an address131.181.114.10/22

The ifconfig & route commands would be ifconfig eth inet 131.181.114.10 netmask

255.255.252.0 broadcast 131.181.115.255

route add net 131.181.112.0 netmask255.255.252.0 dev eth

H d d t i


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How do you determineNetwork and Broadcast

address quickly? There are different subnetting exercises

Given an IP address & mask,

What is the network/subnetwork address

What is the network/subnetwork broadcastaddress

What are the assignable address in thatnetwork/subnetwork

What are all the valid subnet addresses How many nodes per subnet


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Algorithm for deriving

Subnet information Given an IP address, you will usually begiven a net/subnetmask

If you are given the mask Subtract the mask from 256 This is known as the multiplier The first number in each multiplier value

is the network number The broadcast address is the nextmultiplier value subtract 1


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Subnet information Eg given the IP address 192.168.0.100

with the subnet mask of255.255.255.240Or 192.168.0.100/28 What is the network number

What is the broadcast address What are the valid IP hosts for the

subnet


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Subtract 256 from the netmask 256 - 240 = 16 This is the multiplier ie the networks are in steps of 16

(16,32,48,64,96,112 etc)

The IP address 192.168.0.100 is in the range of |100 / 16|which is the 6 th subnetwork

The network address is16* 6 = 96 (01100000b)

The Broadcast address is 96 + 16 - 1

=> 192.168.0.111 (01101111b) ie (next multiplier 1)


Subnet information

Host portion

Subnetwork portion


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Subnet information11000000

1010100000000000

01100100

11111111

11111111 11111111

11110000

11000000

1010100000000000

01100000

11000000 10101000

00000000

01101111

Host portionNetwork Portion SN

IP address

192.168.0.100

Netmask255.255.255.240

Network Address

192.168.0.96

Broadcast Address

192.168.0.111


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Subnet information Valid ranges are

192.168.0.97 to 192.168.0.110

Number of allowable hosts 97 to 110 (incl) = 14 or [24]16 - 2 = 14 Remember you cannot use the first address (network address) and the

last address (broadcast address) in the range The number of allowable networks

[24]16 - 2 = 14 ( ie 4 bits used. If a class B address with the last bit subnet,then add another 8 bits to give you 212 2 allowable subnet)


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Subnet information What if the IP range goes over 2 octets

Use the same principal

Remember octets with all 0s are consideredboring and will be assigned the mask of 0

You will then have to locate the position in theaddress with both 1s and 0s (interesting byte)and use the same algorithm

Similarly all 1s are also considered boring and willbe given the mask of 255 (eg subnetting the lastbyte of a class B address)


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Subnet information Example

QUT students supernet address 131.181.112.0/22

Netmask expanded : 255.255.252.0 Last byte is not interesting hence we set it to 0 for networkand 1 for broadcast

The third byte is interesting256 252 = 4 (multiplier)

Networks are in increments of 4 steps

112/4 = 28 (the 28th subnetwork). Since there is no remainder,it is the beginning of the network address


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Subnet information

Network address :

131.181.112.0 Netmask :

255.255.252.0 Broadcast :

112 + 4 1 = 115=>131.181.115.255

Number of valid hosts :210 2 = 1024 2 = 1022 hosts

Number of subnetworks available for this network26 2 = 64 2 =62 subnetworks

Assignable addresses in this subnetwork131.181.112.1 131.181.115.254

Remember, the 1st and last addressescannot be used (Network and

broadcast)


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handle IP addresses and

subnet masks? Let's assume that a device on another networkwith an IP address of 197.15.22.44 wants to senddata to another device attached to Cisco's

network with an IP address of 131.108.2.2. The data is sent out over the Internet until it

reaches the router that is attached to Cisco'snetwork.

The router's job is to determine which one ofCisco's subnetworks the data should be routed to.


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h n th t p f ms this


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when the router performs thisANDing" operation, the host

portion falls through.

The router looks at what is left which


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The router looks at what is left whichis the network number including the

subnetwork.

h h l k bl


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The router then looks in its routing tableand tries to match the network number

including the subnet with an interface.


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How does the "Anding operation

change with different subnet masks? Imagine that you have a class "B network with

the network number 172.16.0.0.

After assessing the needs of his network, the

network administrator has decided to borroweight bits in order to create subnetworks.

When eight bits are borrowed to create subnets,the subnet mask is 255.255.255.0.

Someone outside the network sends data to theIP address 172.16.2.120.


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Exercise

Exercise You are given an IP address for a host

172.168.35.10/20

What is/are the

1. Subnet address? 2. Broadcast address? 3. The number of useable hosts available

for this subnet?

4. The number of useable subnets availablefor this network? 5. The assignable address range for this

subnet?Answers


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Exercise

Exercise Your organisation has been assigned a class B

IP address of 130.10.0.0 You require about 2000 subnetworks

Work out the 1. Subnet mask required for this subnet 2. The network and broadcast addresses for

the first 5 useable subnets

3. The number of hosts for each subnet 4. The assignable address range of the first5 useable subnets

Answers


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ROUTINGROUTINGROUTINGROUTING


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Routing Routing describes a process wherepackets are forwarded from one

network to another. Routing can be performed by devices

such as :

dedicated routers servers with more than one networkinterface: multihomed hosts

switches incorporating a route function.


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Routers Routing devices typically have morethan one network interface, each

called a port. Routers process datagrams

individually, making routing aprocessing-intensive operation.

Dedicated routers offer betterperformance characteristicscompared with multi-homed hosts.


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Routing Tables To determining the properdestination network for datagrams,

routers consult an internal table. The table consists of records, oneper line, each representing a knownnetwork.

Each record includes a set ofassociated characteristics such asnetmask


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Building Route Tables. Routing table entries can be built bytwo methods: Static: entries are entered manually by

a network administrator Dynamic: entries are entered

dynamically by routing protocols.Routers learn destination networkaddresses by the periodic exchange ofroute tables between routing devices.Routing protocols use IP to deliver thisinformation.


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Configuring Routers Before IP routers can perform the routefunction and use routing protocols toexchange route tables, each interface(port) must be correctly numbered with avalid host IP address and netmask.

The IP address must be selected from

within the range for the particularnetwork address. Typically local gatewaysare located the first address in the validhost range.


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Routing Protocols Routing protocols are used by routers to: learn the location of destination networks.

determine the best route to reach networks.

Examples of routing protocols include: RIP, Routing Information Protocol

OSPF, Open Shortest Path First

BGP, Border Gateway Protocol


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Routing Protocols - 2 Routing protocols differ in: the way in which they exchange route

tables determine the route to the destination

the information that is communicated


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Distance Vector Distance Vector routing protocolsbroadcast the entire route table on a

regular basis. RIP2 typically defaultsat once every 30 seconds.Thiscreates considerable network traffic.

They determine the best route pathon the basis of the least numbernumber of hops to reach adestination network.


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Link State Link state protocols only broadcastchanges to route information after

an initial entire table has been sent. When determining the best path,other factors such as policies (e.g.preferred path) and cost ( timetaken, available bandwidth) caninfluence the choice when multiplepaths are available.


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IP Routing Before a routing device can forwardan IP datagram it must: examine the Destination Address in the

datagram use the netmask to identify the network

portion of the packets destinationaddress

find a corresponding network address inthe route table and forward the packetto the gateway or interface specified


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Route Table Fields To forward IP datagrams, the routeruses the following fields of the the

route table: Destination Network Mask

Gateway


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Destination This field lists the networks which areknown to the router. Addresses may havebeen entered by an administrator, ordynamically learned from thetransmissions of other routers.

Address entries concerned with routing

between network addresses will be of theformat{, 0}


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Netmask The netmask field provides therouter with the ability to determine

the network address of packetsbeing examined.

A logical AND is performed using thenetmask and the destination address.This logically removes the hostportion allowing the router toidentify the destination


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Gateway The Gateway field lists the IP addressesof the interface where the datagramshould be sent (forwarded) to reach the

specified Destination. This field may contain :

An IP interface address corresponding to anadjacent router

0.0.0.0

The address of a interface


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Destination Hop=0.0.0.0 A Destination of 0.0.0.0 indicates adirectly connected network. Hosts

located on this network can bereached using the local networkmethod. If the network is Ethernet,the ARP protocol is used to find the

physical address of the node.

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Sub Netting and Routing