Addressing
By
Tamanna Sait & Aneesha Deo
5/28/2016
1
Introduction
IP Network Addressing

Internet Scaling Problems
Classful IP Addressing
Subnet Addressing
Variable Length Subnet Masks (VLSM)
Classless Inter-Domain Routing (CIDR)
Routing Protocols in Internet



Routing Information Protocol (RIP)
Open Shortest Path First (OSPF)
Border Gateway Protocol (BGP)
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2
IP Network Addressing
Today, the Internet has entered the public
consciousness as the world’s largest public data
network, doubling in size every nine months
There is a direct relationship between the value of
the Internet and the number of sites connected to
the internet
The internet has experienced two major scaling
issues as it has struggled to provide continuous
and uninterrupted growth
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Internet Scaling Problems
The first problem is concerned with the eventual
depletion of the IP address space
The current version of IP, IP version 4 (IPv4),
defines a 32-bit address which means that there
are only 232 (4,294,967,296) IPv4 addresses
available
IP address space has not been efficiently allocated
Traditional model of classful addressing does not
allow the address space to be used to its maximum
potential.
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Internet Scaling Problems
The second problem is caused by the rapid growth
in the size of the Internet routing tables
Internet backbone routers are required to maintain
complete routing information for the Internet
Over recent years, routing tables have experienced
exponential growth
Unfortunately, the routing problem cannot be
solved by simply installing more router memory
and increasing the size of routing tables
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Internet Scaling Problems
Other factors related to the capacity problem
include:



growing demand for CPU horsepower to compute
routing table/topology tables
increasing nature of WWW connections and their effect
on router forwarding caches
volume of information to be managed by people and
machines
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Internet Scaling Problems
The long term solution to these problems can be
found in the widespread deployment of IP Next
Generation or IPv6
Classless Inter domain Routing (CIDR) is a
solution to efficiently utilize the existing address
space
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Classful IP Addressing
When IP was first standardized in Sep 1981, each
system attached to the IP based Internet had to be
assigned a unique 32-bit address
The 32-bit IP addressing scheme involves a two
level addressing hierarchy
Network Number/Prefix
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Host Number
8
Classful IP Addressing
Network number is also referred to as the network
prefix
All hosts on a given network share the same
network prefix but have a unique host number
Two hosts on different networks must have
different network prefixes but may have the same
host number
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Classful IP Addressing
IP address space is divided into 3 different address
classes – Class A, Class B and Class C
Each class fixes the boundary between the
network-prefix and the host number at a different
point in the 32-bit address
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Classful IP Addressing
Class A
01
0
78
31
Class A network has an 8-bit network prefix with
the highest order bit set to 0 and a seven-bit
network number, followed by a 24-bit hostnumber
A maximum of 126 (27 – 2)/8 networks can be
defined
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Classful IP Addressing
Class B
0 2
10
15 16
31
Class B network has a 16-bit network prefix with
the 2 highest order bit set to 1-0 and a 14-bit
network number, followed by a 16-bit hostnumber
A maximum of 16,384 (214)/16 networks can be
defined with up to 65,534 (216 – 2) hosts/network
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Classful IP Addressing
Class C
0 3
110
23 24
31
Class C network has a 24-bit network prefix with
the 3 highest order bit set to 1-1-0 and a 21-bit
network number, followed by a 8-bit host-number
A maximum of 2,097,152 (221)/24 networks can
be defined with up to 254 (28 – 2) hosts/network
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Classful IP Addressing
Class D
0
4
1110
31
Multicast Address
Class D addresses have their leading 4-bits set to
1-1-1-0 and are used to support Multicasting
For example:



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224.0.0.1
224.0.0.2
224.0.0.5
All systems on LAN
All routers on LAN
All OSPF routers on LAN
14
Classful IP Addressing
Class E
0
5
11110
31
Reserved for Future use
Class E addresses have their leading 5-bits set to
1-1-1-1-0
Reserved for experimental/future purpose
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Subnetting
Subnet addressing is used by system
administrators in order to further subdivide an
Internet address within an organization
Instead of the classful two-level hierarchy,
subnetting supports a three-level hierarchy
Network-Prefix
Host-Number
Network-Prefix Subnet-Number Host-Number
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Subnetting
Subnetting attacked the expanding routing
problem by ensuring that the subnet structure of a
network is never visible outside of the
organization’s private network
The route from the Internet to any subnet of a
given IP address is the same, no matter which
subnet the destination host is on
All subnets of a given network number use the
same network-prefix but different subnet numbers
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Subnetting
Extended-Network-Prefix
Routers within the subnetted environment use the
extended-network-prefix to route traffic between
the individual subnets
The extended-network-prefix is composed of the
classful network-prefix and the subnet-number
The extended-network-prefix has traditionally
been identified by the subnet mask
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Subnetting – Example
US
Europe
Subnet B
network #: 00001011.100
11.32.0.0
R
R
R
R
Subnet C
R
Subnet A
11.0.0.0
network #: 00001011.000
11.64.0.0
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Subnetting – Example
Given:
US has 3 locations, each with a router
Europe has 2 locations with routers
Class A IP Address of 11.0.0.0 has been obtained
Need:
To create unique network numbers for each side of
the routed network
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Subnetting – Example
We need to decide which bits of the host address
to use as a part of the network number
Keep all network bits on the left and the host
numbers on the right hand side of the IP address
We use the highest three bits of the host address
area for the subnet mask
The bits of our address are divided into
Network number: nnnnnnnn.ssshhhhh.hhhhhhhh.hhhhhhhh
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Subnetting – Example
The first bit of the subnet will equal 0 if the packet
is to be routed to the US, and 1 if it is to be routed
to Europe
The remaining two bits will be used to
differentiate the routers within the continent
For example, routers in the US will have the
subnet mask values of



000
001
010
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Subnet A
Subnet B
Subnet C
22
Subnetting – Example
The network numbers of the subnets in the US are
00001011.00000000.00000000.00000000 11.0.0.0
00001011.00100000.00000000.00000000 11.32.0.0
00001011.01000000.00000000.00000000 11.64.0.0
Because the subnet bits have been divided
logically based on their routes, it will be easier to
determine which subnet a packet is destined for
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Subnetting – Example
For this example, given Class A address of
11.0.0.0 and a subnet mask of 255.255.0.0 and one
of the workstations in the US had an IP address of
11.1.1.69, the network portion is
11.1.1.69
255.255.0.0
11.1.0.0
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00001011.00000001.00000001.01000101
&
11111111.11111111.00000000.00000000
00001011.00000001.00000000.00000000
24
Subnetting – Example
To find the destination address for a directed
broadcast for net 11.1.0.0, take the ~ of the subnet
mask then bitwise OR it with the IP address
11.1.1.69
00001011.00000001.00000001.01000101
|
0.0.255.255 00000000.00000000.11111111.11111111
11.1.255.255 00001011.00000001.11111111.11111111
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Variable Length Subnet Masks (VLSM)
When an IP network is assigned more than one
subnet mask, it is considered a network with
“variable length subnet masks” since the
extended-network-prefixes have different lengths
VLSM allows the recursive division of an
organization’s address space so that it can be
reassembled and aggregated to reduce the amount
of routing information at the top level
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Classless Inter-Domain Routing (CIDR)
Disadvantages of Classful IP Routing
The near-term exhaustion of the Class B network
address space
The rapid growth in the size of the global
Internet’s routing tables
The eventual exhaustion of the 32-bit IPv4 address
space
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Classless Inter-Domain Routing (CIDR)
CIDR supports two important features that benefit
global Internet routing system
Eliminates the traditional concept of Class A,
Class B and Class C network addresses, which
enables efficient allocation of IPv4 address space
Supports route aggregation where a single routing
table entry can represent the address space of
thousands of traditional classful routes
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Classless Inter-Domain Routing (CIDR)
Efficient allocation of the IPv4 address space
Replaces the traditional concept of Classful
addresses with the generalized concept of a
“network-prefix”
Network-prefix determines the dividing point
between the network and the host number
Supports the deployment of arbitrarily sized
networks rather than the standard 8-bit, 16-bit or
24-bit network numbers associated with Classful
addressing
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Classless Inter-Domain Routing (CIDR)
Each piece of routing information is advertised
with a bit mask or prefix length which specifies
the number of leftmost contiguous bits in the
network portion of each routing table entry
All prefixes with same prefix length represent
same amount of address space
For example, a/20 represents a network with a 20
bit prefix length and 12 bit host number and can
support up to 212 (4096) host addresses
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Classless Inter-Domain Routing (CIDR)
Minimization of Routing table entries
A single routing table entry can specify how to
route traffic to many individual network addresses.
The world is partitioned into 4 zones and each one
is given a portion of Class C address space.
Addresses 194.0.0.0 – 195.255.255.255 -> Europe
Addresses 198.0.0.0 – 199.255.255.255 -> N. America
Addresses 200.0.0.0 – 201.255.255.255 -> C. & S. America
Addresses 202.0.0.0 – 203.255.255.255 -> Asia & Pacific
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Classless Inter-Domain Routing (CIDR)
32 million addresses to allocate, with another 320
million class C addresses from 204.0.0.0 through
223.255.255.255 held in reserve for the future use
The advantage of this allocation is that now any
router outside of Europe that gets a packet
addressed to 194.xx.yy.zz or 195.xx.yy.zz can
send it to its standard European Gateway, thus
reducing the routing table entry to 1 entry
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Classless Inter-Domain Routing (CIDR)
Each routing table entry consists of a base address
and a 32-bit mask
When a packet comes in, its destination address is
first extracted
The routing table is scanned, masking the
destination address and comparing to the table
entry looking for a match
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Classless Inter-Domain Routing (CIDR)
Example
Cambridge Univ (2048) 194.24.0.0 – 194.24.7.255
Oxford Univ (4096)
194.24.16.0 – 194.24.31.255
Edinburgh Univ (1024) 194.24.8.0 – 194.24.11.255
Routing Table Entry
11000010 00011000 00000000 00000000
11111111 11111111 11111000 0000000
11000010 00011000 00010000 00000000
11111111 11111111 11110000 0000000
11000010 00011000 00001000 00000000
11111111 11111111 11111100 0000000
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CIDR Table Entry
Extract the destination IP address.
Boolean AND the IP address with the subnet mask
for each entry in the routing table.
The answer you get after ANDing is checked with
the base address entry corresponding to the subnet
kask entry with which the destination entry was
Boolean ANDed.
If a match is obtained the packet is forwarded to
the router with the corresponding base address
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Routing Algorithms
Routing is the process of forwarding messages
through switching networks
Routing information is stored in Routing Tables
These tables contain the path information as well
as cost
Routing can be decided in two ways


Static Route Selection – routing info provided manually
Dynamic Route Selection – Distance Vector and Link
State Routing
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Routing In The Internet
The Internet can be viewed as a collection of sub
networks or autonomous systems(AS).
Routing in the Internet involves routing within and
between autonomous systems(AS).
Protocol used for routing within the autonomous
system is “Interior Gateway Protocol” which
includes:


Routing Information Protocol(RIP)
Open Shortest Path First Protocol(OSPF)
Protocol used for routing between autonomous
systems is “Exterior Gateway Protocol” or
“Border Gateway protocol”(BGP)
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Routing Information Protocol (RIP)
RIP is a distance-vector protocol
Using RIP, a gateway host (with a router) sends its
entire routing table to its closest neighbor host
every 30 seconds
The neighbor host in turn will pass the information
on to its next neighbor and so on
RIP uses a hop count as a way to determine
network distance
RIP messages are carried in UDP datagrams,
maximum datagram size 512 octets
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Routing Information Protocol (RIP)
Version
0
Address Family
32-bit IP address
Command (1-6)
0
0
0
metric
24 more routes
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Routing Information Protocol (RIP)
Benefits



The only interior gateway protocol that can be counted
on to really run everywhere
Configuring a RIP system requires little effort
RIP uses an algorithm that does not impose serious
computation or storage requirements on hosts and
routers
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Routing Information Protocol (RIP)
Limitations





Does not solve every possible routing problem
The protocol is limited to networks whose longest path
involves 15 hops
The entire routing table is sent every 30 sec, which
increases traffic
The protocol suffers from “counting to infinity”
problem
This protocol uses fixed “metrics” to compare
alternative routes. It is not appropriate for situations
where routes need to be chosen based on real-time
parameters such as measured delay, reliability or load
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Open Shortest Path First (OSPF)
OSPF supports 3 kinds of connections and
networks:



Point-to-point links between exactly two routers
Multiaccess networks with broadcasting
Multicasting networks without broadcasting
OSPF works by abstracting a collection of actual
networks,routers and lines into a directed graph in
which each arc is assigned a cost.
Shortest path is computed based on weights on the
arcs
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Example Network and Graph for OSPF
WAN 1
A
B
LAN 1
B
A
C
F
G
C
4
4
10
D
10
LAN 2
E
F
4
4
2
2
L1
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E
W1
3
2
D
L2
3
G
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Open Shortest Path First (OSPF)
OSPF allows ASes to be divided into numbered
areas, which is a generalization of a subnet
Outside an area, its topology is not visible
Every AS has a backbone area called area 0 and all
other areas are connected to this backbone
All inter-area packet routing takes place via
backbone area or area 0
Each router that is connected to two or more areas
is part of the backbone
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Open Shortest Path First (OSPF)
AS 1
BGP
protocol
connects
the ASes
Backbone
AS 2
Backbone
router
Area
Internal router
AS 3
AS 4
Area
border
router
AS boundary router
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Open Shortest Path First (OSPF)
Using flooding, each router informs all other
routers in its area of its neighbors and its cost
This information allows each router to construct a
graph for its area and compute its shortest path
The backbone routers, in addition to this, accept
information from the Area Border Routers to
compute the best route to every other router
This information is propagated back to Area
Border Routers and advertised within their areas
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Open Shortest Path First (OSPF)
Limitations



It is complex because it divides the AS into a number of
areas
For the traffic to travel between two areas, it must be
first routed to the backbone (area 0). This may cause
non-optimal routes
Although link-state protocols are not difficult to
understand, OSPF muddles the picture with plenty of
options and features
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Border Gateway Protocol (BGP)
BGP is an exterior gateway protocol
BGP has been designed to allow many kinds of
routing policies to be enforced in the interAS
traffic
These policies are manually configured and are
not a part of the protocol
Typical policies involve political, security or
economic considerations. For example


Traffic starting or ending at IBM should not transit
Microsoft
No transit traffic through certain AS systems
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Border Gateway Protocol (BGP)
From the point of view of a BGP router, the world
consists of other BGP routers and the lines
connecting them
Based on transit traffic, networks are grouped into
one of the three categories



Stub networks: Have only one connection to the BGP
graph and cannot be used for transit traffic
Multi-connected: Could be used for transit traffic unless
they refuse
Transit Networks (Backbones): These networks are
willing to handle third party packets with some
restrictions
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Border Gateway Protocol (BGP)
BGP is fundamentally a distance vector protocol
with some modifications
Instead of maintaining a cost to each destination,
each BGP router keeps a track of the exact path
used
BGP peers initially exchange their full routing
tables
Thereafter, they exchange routing updates only
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Border Gateway Protocol (BGP)
When a BGP router wants to forward a particular
packet to a destination, it examines all the paths
from its neighbors to that same destination



Paths passing through the BGP router itself are scored
out
Paths violating a policy constraint are also
automatically scored out
From the remaining paths, the router selects the shortest
distance
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Border Gateway Protocol (BGP)
The routing information exchanged between BGP
peers is in the form of routing updates, includes



Network number
List of AS that routing information has passed through
The list of path attributes
BGP router employs a BGP speaker which is an
entity within the router that transmits and receives
BGP messages and acts upon them
The messages types are open, update, notification
and keep alive
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BGP Packet Format
A BGP packet format consists of four fields
16
Marker




2
1 variable
Length Type Data
Marker: Authentication value that the message receiver
can predict
Length: Total length of message in bytes
Type: Specifies the message type
Data: Optional field
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BGP Message types
Open Message Format






Version [1 byte]: Provides BGP version number
Autonomous System [2 bytes]: Provides AS number of
the sender
Hold Time [2 bytes]: The amount of time the message
remains valid
BGP Identifier [4 bytes]: Provides the BGP identifier of
the sender (IP address of the sender)
Optional Parameter Length [1 byte]: Length of the
optional parameter field if present
Optional Parameters [4 bytes]: Authentication info
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BGP Message types
Update Message Format





Unfeasible Routes Length [2 bytes]: Total length of the
withdrawn routes field or the absence of the routes
Withdrawn Routes: [Variable]: Contains a list of IP
address prefixes for routes withdrawn from service
Total Path Attribute Length [2 bytes]: Total length of
path attribute field or absence of the field
Path Attribute [Variable]: Describes the characteristics
of the advertised path and helps in decision making
Network Layer Reachability Information: Contains list
of IP address prefixes for advertised routes
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BGP Decision Algorithm
When BGP speaker receives updates from
multiple ASs that describe different paths to the
same destination, then the single best path is
chosen using the various list attributes







AS-path Attribute
Origin Attribute
Next Hop Attribute
Weight Attribute
Local Preference Attribute
Multi-Exit Discriminator Attribute
Community Attribute
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BGP List Attributes in Update Message
Format
AS-path Attribute

All the AS numbers that an update has traversed to
reach a destination
Origin Attribute

It is generated by the AS that originates the associated
routing information
Next Hop Attribute

IP address of the next hop that is going to be used to
reach a destination
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BGP List Attributes in Update Message
Format (contd)
Weight Attribute


Used in path selection process when there is more than
one route to the same destination
It is local to the router and not propagated in routing
updates
Local Preference Attribute


It indicates the preferred path when there are multiple
paths to the same destination
Higher preference path is preferred
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BGP Example
170.10.0.0
200
150
AS100
AS300
AS213
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BGP List Attributes in Update Message
Format (contd)
Multi-Exit Discriminator Attribute


It is a hint to the external neighbors about the preferred
path into an AS when there are multiple entry points
into the AS
Lower MED value is preferred
Community Attribute

Provides a way of grouping destinations to which
routing decisions can be applied
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BGP Message types (contd)
Notification Message Format



Error Code [1 byte]: Indicates the type of error
occurred, such as Message Header Error, Open
Message Error, Update Message Error, Hold Time
Error, Finite State Machine Error and Cease
Error Subcode [1 byte]: Provides more specific
information about the nature of the reported error
Error Data [1 byte]: Contains data based on the error
code and error subcode
Keep Alive Message Format

Used to maintain the established connection
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Comments on Routing Protocols
RIP


Simple and suitable for small networks
High overhead
OSPF


Avoids the problems caused by RIP
Complex
BGP


Used in Internet Backbone Routers
Takes into account various policies like political,
economc or security
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Comments on BGP and EGP
BGP-4 supports CIDR whereas EGP assumes that
each advertised network is a natural Class network
based on its higher order bits
EGP-2 was used in old internet topology which
was small, simple, 2-tier model with core AS and
additional ASs around
AS numbers in EGP could not exceed 16-bits (165535)
EGP-2 cannot support a hop count greater than
255
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References
1.
2.
3.
4.
5.
6.
http://www.alexia.net.au/~www/yendor/internetinfo/subnet_adrs.html
http://www.sangoma.com/fguide.htm
http://sifu.rindu.net/course/one/freesoft/CIE/RFC/1812/20.htm
http://www.zdnet.co.uk/pcmag/labs/1999/01/nos/5.html
http://www.library.ucg.ie/Connected/Topics/90.htm
http://sunsite.net.edu.cn/tutorials/NetworkingGuide/BOOKCHAPTER5.html
7. http://www-win.uniinc.msk.ru/tech1/index.htm
8. http://wwwsop.inria.fr/rodeo/personnel/eduros/benchtests/nettopo.html
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