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Chapter 19 + 22
Addressing and Routing
An IPv4 address is a 32-bit address that uniquely
and universally defines the connection of a device
(for example, a computer or a router) to the Internet.
Topics discussed in this section:
Address Space
Notations
Classful Addressing
Classless Addressing
Network Address Translation (NAT)
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IPv4 ADDRESSES
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Two devices in the Internet can never have the same
address at the same time.
An address may be assigned to a device for a time period
and then taken away and assigned to another device.
If a device operating at the network layer (e.g. router) has
m connections to the Internet, it needs to have m IP
address.
The IPv4 addresses are unique and universal
IPv4
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IPV4 has an address space: is the total number of
addresses used by the protocol.
If a protocol uses N bits to define an address, the address
space is 2N .
IPv4 uses 32-bit addresses:
 The address space=232 =4,294,967,296 ( more than 4 billion)
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This means, if there were no restrictions, more than 4 billion
devices could be connected to the Internet.
IPv6 uses 128 bit-addresses
IPv4 Addresses: Notations
There are two prevalent notations to show an IPv4 address
 Binary notation
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Dotted-decimal notation:
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Address is displayed as 32 bits.
Each octet is often referred to as byte.
IPv4 address referred to as 32-bit address or 4- byte address
More compact and easier to read
Written in decimal form with a decimal point( dot) separating the
bytes
Example: 117.149.29.2
Each decimal value range from 0 to 255
Address Allocation
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How are the block allocated?
The address allocation is given to global authority
called
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Internet Assigned Number Authority (IANA)
IANA does not allocate addresses to individual organizations
It assigns a large block of addresses to an ISP
Example
Change the following IPv4 addresses from binary
notation to dotted-decimal notation.
Solution
We replace each group of 8 bits with its equivalent
decimal number (see Appendix B) and add dots for
separation.
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Example
Change the following IPv4 addresses from dotted-decimal
notation to binary notation.
Solution
We replace each decimal number with its binary
equivalent (see Appendix B).
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Example
Find the error, if any, in the following IPv4 addresses.
Solution
a. There must be no leading zero (045).
b. There can be no more than four numbers.
c. Each number needs to be less than or equal to 255.
d. A mixture of binary notation and dotted-decimal
notation is not allowed.
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Classfull addressing
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In classfull addressing, the address space is divided into five
classes: A, B, C, D, and E
We can find the class of an address in
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Binary notation: the first few bits define the class
Decimal-dotted notation: the first byte define the class
Classfull addressing
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In classfull addressing, the address space is divided
into 5 classes: A, B, C, D, and E.
Addresses in Classes A, B and C are uniast ddresses
A host needs to have at least one unicast address to
be able to send packet (Source).
Addresses in Class D are for multicast address
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Used only for destination
Addresses in class E are reserved
Example 19.4
Find the class of each address.
a. 00000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
c. 14.23.120.8
d. 252.5.15.111
Solution
a. The first bit is 0. This is a class A address.
b. The first 2 bits are 1; the third bit is 0. This is a class C
address.
c. The first byte is 14; the class is A.
d. The first byte is 252; the class is E.
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Classfull Addressing
Classes and Blocks
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Net Id and Host Id
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The address is divided into Netid and Hostid.
These part are of varying lengths, depending on the class.
Dose not apply to classes D and E
Classfull Addressing
Classes and Blocks
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Classes and Blocks
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Class A address
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Class B address
designed for midsize organizations with ten of thousands of attached
hosts or routers( too large for many organizations)
Class C address
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designed for multicasting. (waste of addresses)
Class E address
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designed for small organizations with a small number of attached
hosts or routers (too small for many organizations)
Class D address
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designed for large organizations with a large number of attached
hosts or routers. (most of the addresses were wasted and not used)
reserved for future use (waste of addresses)
One problem is that each class is divided into fixed
number of blocks with each block having a fixed size

Mask (default mask)
 Help us to find the NetId and HostId
 Mask: 32-bit made of 1s followed by 0s.
 Dose not apply to classes D and E.
 CIDR(Classless Interdomain Routing): used to show
the mask in the form /n (n=8,16,24)
Classfull addressing, which is almost obsolete, is replaced
with classless addressing.
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Classfull Addressing
Network address
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The network address is an address that define the
network itself to the reset of the internet
The network address has the following properties:
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16
All hostid bytes are 0’s
It is the first address in the block
It cannot be assigned to a host
Given the network address, we can find the class of the
address
Example
Find the network address for the following
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132.6.17.85
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The class is B
The first 2 bytes defines the Netid. We can find the network address
by replacing the hostid bytes (17.85) with 0s
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Therefore, the network address is 132.6.0.0.
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23.56.7.91
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The class is A. Only the first byte defines the Netid. We can find the network address
by replacing the hostid bytes (56.7.91) with 0s.
Therefore, the network address is 23.0.0.0
Figure 19.3 A block of 16 addresses granted to a small organization
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In IPv4 addressing, a block of addresses can be
defined as x.y.z.t /n
x.y.z.t defines one of the addresses and the /n
defines the mask
The first address in the block can be found by
setting the rightmost
32 − n bits to 0s
The last address in the block can be found by
setting the rightmost
32 − n bits to 1s.
Example
A block of addresses is granted to a small organization. We
know that one of the addresses is 205.16.37.39/28.
1. What is the first and the last address in the block
2. Find the number of addresses?
Solution
The binary representation of the given address is
11001101 00010000 00100101 00100111
If we set 32−28 = 4 rightmost bits to 0, we get
11001101 00010000 00100101 00100000
205.16.37.32
If we set 32−28 = 4 rightmost bits to 1, we get
11001101 00010000 00100101 00101111
 The number of addresses is 2 32−28 = 16
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A network configuration for the block 205.16.37.32/28
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The first address in a block is normally not assigned to
any device; it is used as the network address that
represents the organization to the rest of the world
The Last address in a block is normally not assigned to
any device; it is used as the Broadcast address
Configuration and addresses in a subnetted network
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Each address in the block can be considered as a
two-level hierarchical structure
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the leftmost n bits define the network
the rightmost 32 − n bits define the host.
Three-level hierarchy in an IPv4 address
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Example
An ISP is granted a block of addresses starting with
190.100.0.0/16 (65,536 addresses). The ISP needs to distribute
these addresses to three groups of customers as follows
a. The first group has 64 customers; each needs 256 addresses.
b. The 2nd group has 128 customers; each needs 128 addresses.
c. The 3rd group has 128 customers; each needs 64 addresses.
Design the subblocks and find out how many addresses are still
available after these allocations.
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Solution
Group 1
For this group, each customer needs 256 addresses. This means that
8 bits are needed to define each host. The prefix length is 32 − 8 = 24
Group 2
For this group, each customer needs 128 addresses. This means that
7 bits are needed to define each host. The prefix length is 32 − 7 = 25
25
Group 3
For this group, each customer needs 64 addresses. This means that 6
bits are needed to each host. The prefix length is 32 − 6 = 26
Number of granted addresses to the ISP: 65,536
Number of allocated addresses by the ISP: 40,960
Number of available addresses: 24,576
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Classfull Addressing
Subnetting
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If an organization was granted a large block in classes A or B
It could divide the addresses into several contiguous groups
and assign each group to smaller networks ( subnets)
It increases the number of 1s in the mask
To make a subnet mask , we change some of the leftmost 0s in
mask to 1s
The number of subnets is determine by the number of extra1s.
If the number of extra 1 is n, the number of subnets is 2n
Example
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Class B address
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For 4 subnets : (need 2-extra bits )
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Subnet mask: 255.255. 192.0 or /18
11111111 11111111 11 000000 00000000
For 8 subnets: (need 3-extra bits )
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mask : 255.255.0.0 or /16
11111111 11111111 00000000 000000000
subnet mask : 255.255.224.0 or /19
11111111 11111111 111 00000 00000000
Example
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A router receives a packet with destination address
190.240.33.91.
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Show how it finds the network and the sub_network address to route
the packet.
Assume the subnet mask is /19
The router follows steps:
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The router looks at the first byte of the address to find the class. It is class B
The mask for class B is (/16)The router ANDs this mask with the address to get
the network address :
190.240.0.0
The router applies the subnet mask (/19) to the address, 190.240.33.91:
ı
190.240.001 00001.91
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The subnet address is 190.240.32.0
The router looks in its routing table to find how to route the
packet to this destination
Supernetting
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Huge demand for midsize blocks.
Although class A and B addresses are almost depleted,
class C addresses are still available( size of block= 256
address did not satisfy the needs).
In super netting, an organization can combine several
class C blocks to create a larger range of addresses.
Several networks are combined to create a super network
( super net).
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e.g. Organization needs 1000 address can be granted 4 contiguous
class C blocks to create one super network.
Subnetting
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Collision domain
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Broadcast domain
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Are the connected physical network segments where collisions can
occur
A group of collision domains that are connected by layer 2 devices
Collision domains = # of hosts connected to a switch or bridge + # of router links
Broadcast domains = # of router links, since only routers will create broadcast domains
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A BRIDGE will create a collision
domain while the HUB will not
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Network Address Translation
NAT
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The Internet Authorities have reserved 3 sets of addresses as a private
addresses
Network Address Translation
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NAT enables a user to have a large set of addresses internally and
one address or small set of address externally.
A NAT box located where the LAN meets the Internet makes all
necessary IP address translations
Any organization can use an addresses out of this set without
permission from internet authorities.
Provides a type of firewall by hiding internal IP addresses
Network Address Translation
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Private addresses are unique inside the organization , but they are not
unique globally
No router will forward a packet that has one of these addresses as the
destination addresses
The site must have one connection to the global internet through the
router that runs NAT software
The router has uses one private address and one global address
The internet sees only NAT router with global address
NAting
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NAT router replaces the source address in the outgoing
packets with the global NAT address (200.24.5.8)
Router replaces the destination address (the NAT router
global address) in the incoming packets with appropriate
private address
NAT address translation
Note
private network must start (initiate ) the communication
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NAT address translation
Using pool of IP addresses
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Restriction
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For example
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Since the NAT router has only one global address, only one private
network host can
To remove the previous restriction, the NAT router uses a pool of
global addresses access the same external host
instead of using one global address 200.24.5.8 , the NAT router can
uses 4 addresses (200.24.5.8, 200.24.5.9, 200.24.5.10, 200.24.5.11).
In this case 4 private network hosts can communicate the same
external host at the same time because each pair of addresses
defines a connection
Pool of IP addresses
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Pool of IP addresses
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There are still some drawbacks:
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No than four private network hosts can communicate the same
destination
No private network host can access two external server programs
(e.g. HTTP (port 80 )and FTP) at the same time
To allow many to many relationship use 5- coloum translation
table
NAT address translation
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Using both IP addresses and Port number
Five-column translation table
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IPv6 ADDRESSES
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Despite all short-term solutions, address depletion is still a
long-term problem for the Internet. This and other problems
in the IP protocol itself have been the motivation for IPv6.
An IPv6 address is 128 bits long
IPv6 address in binary and hexadecimal colon notation
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Abbreviated IPv6 addresses
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Example
Expand the address 0:15::1:12:1213 to its original.
Solution
We first need to align the left side of the double colon to
the left of the original pattern and the right side of the
double colon to the right of the original pattern to find
how many 0s we need to replace the double colon.
This means that the original address is.
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Routing
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Routing
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Packet go from source to destination via routers.
Router consults the routing table.
Routing table can be
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Routing protocols are needed to create the routing tables
dynamically.
A routing protocol is a combination of rules and
procedures that
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Static: does not change automatically (manual entries)
Dynamic: updated automatically when there is change in the
Internet
Lets routers in the internet inform one another of changes.
Allows routers to share whatever they know about the internet
or their neighborhood.
Route method vs Next-hop method
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Host-specific vs Network-specific method
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Host-specific routing
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Default method
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Simplified forwarding module in classless
address
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In classless addressing, we need at least 4 columns in a
routing table
Example
Make a routing table for router R1, using the following configuration
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Example
Show the forwarding process if a packet arrives at R1 with
the destination address 180.70.65.140
Routing table for R1
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Solution
The router performs the following steps
1. The first mask (/26) is applied to the destination address
 The result is 180.70.65.128, which does not match the
corresponding network address
2. The second mask (/25) is applied to the destination address.
 The result is 180.70.65.128,which matches the corresponding
network address. The next-hop address and the interface number
m0 are passed to ARP for further processing
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Example
Show the forwarding process if a packet arrives at R1 in with
the destination address 201.4.22.35
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The router performs the following steps
1. The first mask (/26) is applied to the destination address
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The result is 201.4.22.0, which does not match the corresponding
network address
2. The second mask (/25) is applied to the destination address
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The result is 201.4.22.0, which does not match the corresponding
network address (row 2)
3. The third mask (/24) is applied to the destination address
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The result is 201.4.22.0, which matches the corresponding network
address. The destination address of the packet and the interface
number m3
Example
Show the forwarding process if a packet arrives at R1 with
the destination address 18.24.32.78
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This time all masks are applied, one by one, to the
destination address, but no matching network address is
found. When it reaches the end of the table, the module
gives the next-hop address 180.70.65.200 and interface
number m2 to ARP. This is probably an out going
package that needs to be sent, via the default router, to
some place else in the Internet.
CLASSFUL ADDRESSING ROUTING TABLE
Example, the router receives a packet for destination 192.16.7.1.
For each row, the mask is applied to the destination address until a match with the
destination address is found. In this example, the router sends the packet through
interface m0 (host specific).
Example, the router receives a packet for destination 193.14.5.22. For each row,
the mask is applied to the destination address until a match with the next-hop
address is found. In this example, the router sends the packet through interface m2
(network specific).
Example, the router receives a packet for destination 200.34.12.34. For each row,
the mask is applied to the destination address, but no match is found. In this
example, the router sends the packet through the default interface m0.
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Autonomous systems
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Autonomous System (AS)
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Group of networks and routers under the authority of a single
administration.
Routers inside an autonomous system is referred to as
interior routing (Intradomain).
Routing between autonomous systems is referred to as exterior
routing (Interdomain)
Solid lines show the communication between routers that use interior
routing protocols.
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Broken lines show the communication between routers that use an
exterior routing protocols.
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Interior and Exterior routing protocols
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Each AS can choose one or more intradomain (interior)
routing protocol to handle routing inside the AS such as RIP
and OSPF
One interdomain (exterior) routing protocol is usually chosen
to handle routing between ASs ; BGP
Metric of different protocols
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Metric is the cost assigned for passing through a network.
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RIP (Routing Information Protocol) “Shortest distance”
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Cost of passing each network is same; it is one hop count.
If a packet passes through 10 networks to reach the destination, the total
cost is 10 hop counts.
OSPF(Open Shortest Path First)
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The total metric of a particular router is equal to the sum of the metrics
of networks that comprise the route.
A router chooses the route with smallest metric.
The metric assigned to each network depends on the type of protocol
Administrator can assign cost for passing a network based on type of
service required such as :throughput, delay,..etc.
OSPF allows each router to have more than one routing table based on
required type of service
BGP(Border Gateway Protocol)
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Criterion is the policy, which is set by the administrator
Routing Algorithm classification
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Routing algorithms can be
1. distance vector algorithms
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router knows physically-connected neighbors, link
costs to neighbors, iterative process of computation,
exchange of partial information with neighbors.
The least cost between any two nodes is the route
with minimum distance
RIP is an implementation of this approach
2. link state algorithms
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all routers have complete topology, link cost
information
OSPF is an implementation of this approach
Distance Vector Routing
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Each node( router) maintains a set of triples (table):
Destination, Cost, Next Hop)
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Directly connected neighbors exchange updates
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Node knows the cost to each neighbor (the distance between itself
and its immediate neighbors)
periodically (on the order of several seconds -30s)
whenever table changes (called triggered update)
Each update is a list of pairs: Destination, Cost
Update local table if receive a “better” rout (smaller cost)
Distance vector routing
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infinite ∞ ( unreachable).
Think the node as the cities and the lines as the roads connecting them
Distance vector routing
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Sharing: Updating in distance vector routing
Distance vector routing
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The least cost route between any two nodes is the route with
min distance.
Each node maintains a table which contains
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Destination, Cost, Next hop
Distance vector routing
1. Each router shares its entire routing table with its neighbors
2. Sharing
 Periodically update :on the order of several seconds -30s Triggered update: The change can result from the following
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A node receives a table from a neighbor, resulting in changes in its
own table after updating.
A node detects some failure in the neighboring links which results
in a distance change to infinity ∞
3. Each update is a list of pairs
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Destination, Cost(two column routing table)
Routing Information Protocol
RIP implement Distance vector routing with some considerations
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Destination in the routing table is a network (first column
defines network address)
Metric(distance) is Hop count : is the number of networks
that a packet encounters to reach its final destination
Infinity is defined as value of 16
Therefore, the Max limited of hops is 15
suitable for small networks (local area environments)
Router sends update message to neighbors every 30 sec.
If router does not receive update message from neighbor X
within this limit, it assumes the link to X has failed and sets
the corresponding minimum cost to 16 (infinity)
In distance vector routing, each node shares its routing table with
its immediate neighbors periodically and when there is a change
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RIP
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Routing table
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Has one entry for each destination network of which the router is
aware
Each entry has destination network address, the shortest distance to
reach the destination in hop count, and next router to which the
packet should be delivered to reach its final destination
Example
Internetwork
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Initial routing tables in a small AS
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When a router is added to a network, it initializes a routing
table for itself, using its configuration file
The table consists only the directly attached networks and the
hop counts, which are initialized to 1
The next-hop field, which identifies the next router, is empty
Final routing tables
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Each routing table is updated upon receipt of RIP messages
using the RIP updating algorithm
RIP Updating Algorithm
Receive: a response RIP message(significant portion of its
routing table)
1. Add one hop to the hop count for each advertised
destination
2. Repeat the following steps for each advertised destination

 If (destination not in the routing table)
 Add the advertised information to the table
 Else
 If (next-hop field is the same)
 Replace entry in the table with the advertised one.
 Else
 If (advertised hop count smaller than one in the table)
 Replace entry in the routing table
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Example of updating a routing table
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Example
Domain using RIP
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Link State Routing
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Each node in the domain has the entire topology of the domain
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Analogous to a city map
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Link State Routing
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Link-state routing works by having the routers tell every
router on the network about state of its neighbors.
1) Sharing knowledge about the neighborhood
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Each router sends the state of its neighborhood to every other router in
the area.
2) Sharing with every other router
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By flooding, a process whereby a router sends its information to all its
neighbors (through all its output ports).
Each neighbor sends the packet to all its neighbors, and so on.
Every router that receives the packet sends copies to each of its
neighbors. Eventually, every router (without exception) has received a
copy of the same information
3) Sharing when there is a change; Only to its neighbors
4) The node can use Dijkstra Algorithm to build a routing table
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Metric
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Administrator can assign the cost to each route.
Based on type of service (minimum delay, maximum throughput, ..)
Routing Data Distribution
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LSA-Updates are distributed to all other routers via Reliable
Flooding
Example: Flooding of LSA from 10.10.10.1
10.10.10.1
10.10.10.2
LSA
ACK
10.10.10.4
LSA
Update
database
10.10.10.6
LSA
ACK
Update
database
Update
database
LSA
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Update
database
Update
database
10.10.10.2
10.10.10.5
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OSPF
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OSPF Based on Link state Routing
OSPF divides an AS into areas.
Special routers called autonomous system boundary routers are
responsible for dissipating information about other ASs into the current
system
OSPF
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Areas in an Autonomous System
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Area is a collection of networks, hosts, and routers all
contained within an autonomous system.
Routers inside an area flood the area with routing information.
Area border routers
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Backbone area [Primary area]
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Summarize the information about the area and send it to other routers
All the areas inside an autonomous system must be connected to the
backbone
Routers in this area are called as backbone routers. This area
identification number is 0.
If, due to some problem, the connectivity between a backbone
and an area is broken, a virtual link between routers must be
created by the administration to allow continuity of the
functions of the backbone as the primary area
OSPF
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Each router should have the exact topology of the internet
at every moment (LSP).
From this topology, a router can calculate the shortest path
between itself and each network using Dijkstra algorithm
Point-to-point Link
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Connects 2 routers without any other router or host in between
Directly connected routers using serial line.
Only one neighbor.
No need to assign a network address to this type of link
The metrics are the same at the two ends
Transient link
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A network with several routers attached to it.
Each router has many neighbors.
Lot of advertisements about their neighbors.
One of the routers in the network has two duties
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true router and designated router (network)
Each router has only one neighbor, the designated router
(network). On the other hand, the designated router (network)
has five neighbors.
Designated router represents a network. There exists a metric
between each node to the designated router but there is no
metric from the designated router to any other node.
Stub Link

A network that is connected to only one router
The data packets enter the network through this single
router and leave the network through this same

Virtual Link


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When the link between two routers is broken, the administration
may create a virtual link between them, using a longer path that
probably goes through several routers.
Example of an internet & Graphical
representation
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Dijkstra Algorithm


Calculates the shortest path between two points on a
network, using a graph made up of nodes and edges.
Algorithm divides the nodes into two sets


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tentative and permanent
It chooses nodes, makes them tentative, examines them, and if they
pass the criteria, makes them permanent
Dijkstra Algorithm
1. Start with the local node (router): the root of the tree.
2. Examine each neighbor node of the node that was the
last permanent node
3. Assign a cost of 0 to this node and make it the first
permanent node
4. Assign a cumulative cost to each node and make it
tentative
5. Among the list of tentative nodes
i.
ii.
Find the node with the smallest cumulative cost and make it
permanent
If a node can be reached from more than one direction. Select the
direction with the shortest cumulative
6. Repeat steps 3 to 5 until every node becomes permanent
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Example 1
Shortest-path calculation using Dijkstra
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Example 1
Shortest-path calculation using Dijkstra
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Example 1
Shortest-path calculation using Dijkstra
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Example 1
Shortest-path calculation using Dijkstra
Routing Table for Router A
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Example 2
Shortest-path calculation using Dijkstra
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Example 2
Shortest-path calculation using Dijkstra
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Example 2
Shortest-path calculation using Dijkstra for Router A
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OSPF
Reaction to Failure

If a link fails,



Router sets link distance to infinity & floods the
network with an update packet
All routers immediately update their link database
&recalculate their shortest paths
Recovery very quick
NOTE



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OSPF differs from RIP in that each router knows the exact topology
of the network
OSPF reduces routing bandwidth usage
OSPF is faster than RIP
A physical link is dedicated between source
and destination
Data is sent out as stream of bits
No packetization
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Data are transmitted in discrete units
(packets)
Datagram approach
Each packet is treated independently of all others
Packet here is called datagram
Datagrams might arrive at destination out of order
Used in the Internet
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Virtual Circuit




Relationship between all packets belonging to a
message is preserved
A single route is chosen between sender and
receiver at the beginning of the session
Data are sent one after another
WAN uses this approach which needs


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0
Call setup to establish VC between sender and receiver
Call teardown to delete the VC
DHCP




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1
Dynamic Host Configuration Protocol
Designed to provide information dynamically
It is a client-server program
Used to assign addresses to hosts dynamically
ARP:
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