Integrated Network Services

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24 Οκτ 2013 (πριν από 3 χρόνια και 1 μήνα)

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Integrated Network Services

Network Design

Almerindo Graziano

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Issues in Network Design


Designing an addressing scheme with IP4


Classful vs Classless addressing


How to choose and addressing scheme


How to choose an adequate routing protocol


Examples


Recap


Designing a new Network


How many networks do currently exist?


How many will exist in the next 3
-
12 months?


How many hosts on each network?


How many will exist in the next 3
-
12 months?


Is the number constant or does it vary with the
network?


Does the logical topology reflect the physical
topology?



Designing a new Network


Do you have a client/server environment?


Where are the server located?


Do you need access every network from every
network?


Any security issues?


Any mobile users?


Do you need Internet access?


Do you have your own NIC address or are you
connected via an ISP


Which network protocol will you be adopting?

Designing an Addressing Scheme
with IP


Current version is IP4


32
-
bit long


2
32

binary combinations


4billion


3 classes of addresses in use (Classful)


Class A


1.0.0.0 to 127.255.255.255


Class B

128.0.0.0 to 191.255.255.255


Class C


192.0.0.0 to 223.255.255.255


2 additional classes


Class D


224.0.0.0 to 239.255.255.255


Class E


240.0.0.0 to 247.255.255.255


IP Classful Addresses

2
7
-
2 =
126

Networks

2
24
-
2=
16,777,214

hosts

2
21
=
2,097,152

Networks

2
8
-
2=
254

Hosts

2
14

=
16,384

Networks

2
16
-
2 =
65,534

Hosts

Address Notation


An IP address can be written in two formats


143.52.57.32



10001111.00110100.00111001.00100000
Binary

Dotted Decimal

IP4 addresses


Addresses allocated by IANA


APNIC (Asia
-
Pacific)


ARIN (American Registry for Internet Numbers)


RIPE (Reseu IP Europeens)

Limitations


Lack of support for medium
-
size organization


Example: 300 hosts needed a Class B address


Rapid depletion of class B addresses


Large routing tables


No way to summarize large number of addresses


Routers DO need to know how to reach each network



Next Step: Subnetting


How can we use our network address
efficiently?


RFC 950 (subnetting) : use some of the host
bits to represent a network


More freedom and flexibility within an
organization (no need to speak to IANA)


Increased complexity within the organization


More stable routing tables (reduced flapping)


Still large routing tables

CIDR


What do we do if we need 1600 addresses?


CIDR (Classless InterDomain Routing)


Why Classless


The network is not identified by the Class (first
octect rule)


The network is identified by a subnet mask


The subnet mask identifies the boundary
between the network part and the host part

CIDR


A company needs 1600 address


In the old days:


1600/254 = 6.29


7 Class C addresses


7 Class C addresses


7 new entries on the Internet


Alternatively use 1 Class B address (What a waste!!)


Using CIDR


IANA releases 8 Class C addresses


8 Class C addresses


1 new entry on the Internet

CIDR example


RIPE will provide this address

200.128.48.0/21 The subnet mask is 255.255.248.0


Class C address

110010000.1000000.00110 000 .00000000 = 200.128.48.0






001 .00000000 =
200.128.49.0






010 .00000000 =
200.128.50.0






011 .00000000 =
200.128.51.0






100 .00000000 =
200.128.52.0






101 .00000000 =
200.128.53.0






110 .00000000 =
200.128.54.0






111 .00000000 =
200.128.55.0



8 Class C addresses are allocated to the company


Only 7 are used


Only one new entry is advertised to the internet

CIDR Example

CIDR


Summarization


Smaller routing tables


Less Memory


Smaller routing tables


Less CPU


Smaller updates


Less traffic


More stable routing tables (not always good)


Unecessary traffic can traverse the network for a
while


Classless Routing


What happens if we decide to use different
subnet mask? (Networks are different!!)


First routing protocols didn’t send the subnet mask
(RIP1)


The mask was configured locally


No masks are associated with entries in the routing table


It is assumed that the same mask is used on the major
network


The router has no way of knowing how many bits are
used for the host


SHU uses 143.52.0.0/24

Classful Routing


Classful lookup: a packet arrives at in interface


1)

The network portion of the destination address is



read


2) If the there is no match for the major networks (A,



B, C) the packet is dropped (ICMP)


3) A match is found for a major network



-

All the subnets listed for that network are


examined




-

A match is found and the packet is routed




-

A match is not found and the packet is




dropped


The destination address is interpreted according to
the subnet masks locally configured on the router


Classful and Classless Routing
Protocols


Classless routing protocols send the subnet mask
in their route advertisements


Benefits


All
-
zeros and All
-
ones subnets can be used


143.52.0.0/24

is an all
-
zeros subnet


143.52.0.0/16

is the major network number


It is possible to use VLSM (Variable Length Subnet
Mask)


It is possible to summarize a group of major networks
with one aggregate address


A classful routing protocol doesn’t advertise
routes between interfaces whose mask don’t match



Classful and Classless Routing
Protocols


Examples of Classful routing protocols


RIP1


IGRP


EGP


Examples of Classless routing protocols


RIP2


OSPF


EIGRP


BGP4

Classless Routing


Classless lookup: a packet arrives at an interface


The router doesn’t pay attention to the class of the
destination address


The router performs a bit
-
by
-
bit comparison
between the destination address and all the routing
entries


Classless Routing: Example


A router has the following routing table



D 192.168.32.0/26 [90/25789217] via 10.1.1.1

R 192.168.32.0/24 [120/4] via 10.1.1.2

O 192.168.32.0/19 [110/229840] via 10.1.1.3




A packet arrives destined to 192.168.32.1


Which network will it be forwarded to?




.

.

.

.

.

.

VLSM


Using a single subnet mask has disadvantages


Inefficient use of address space


No summarization possible


Variable Length Subnet Mask (VLSM)


Recursive division of an address space


Allows route aggregation


Efficient use of the address space


Requires new
-
generation protocols (RIP2, OSPF,


EIGRP etc.)

Rules for VLSM


A subnet can be used for


Address host


Further subnetting


It is possible to use the all
-
zeros and all
-
ones
subnets


The rule has to be obeyed only once!


Usually use the least significant bits

VLSM: Example


A organization is spread across


England, Ireland, Scotland and Wales


In each of these countries the organization has a
presence in no more than 3 towns


In each town the company has max 4 buildings
(the number may increase)


No building has more than 4 floors


No floor has more than 20 hosts


The organization is assigned 143.52.0.0


VLSM: Example


Step 1


Identify the number of bits we need for
identifying Countries, Towns, Buildings etc.


We can play with 16 bits


Host


Floor


Building


Town


Country



00110 010.100 00011


VLSM: Example


Step 2


Identify where we will apply the subnet rule


We will use the floor bits


Step 3


Identify addresses for WAN connections


Example: borrow a subnet from available ones:


4 country Subnets


1 town subnet per country


2 floors per building


Document the choice made

VLSM: Example

NIC Number

143.52.0.0


Country
: 000

143.52.0.0/19


Ireland


001

143.52.32.0/19

Scotland


010


143.52.64.0/19

Wales


011


143.52.96.0/19

England


100


143.52.128.0/19 not used


101


143.52.160.0/19

not used


110


143.52.192.0/19

not used


111


143.52.224.0/19

not used

VLSM: Example

NIC Number

143.52.0.0


Town
:

011/00


143.52.96.0/21

London


/01


143.52.104.0/21

Sheffield


/10


143.52.112.0/21

Birmingham


/11


143.52.120.0/21

not used

Building

011/01/000

143.52.104.0/24 Owen



/001

143.52.105.0/24

Howard



/010

143.52.106.0/24

Harmer



/011

143.52.107.0/24

Stoddart



/100

143.52.108.0/24

For future use



/101

143.52.109.0/24

For future use



/110

143.52.110.0/24

For future use



/111

143.52.111.0/24

For future use

VLSM: Example

NIC Number

143.52.0.0


Floors in the Harmer building

011/01/010./000

143.52.106.0/27

Cannot be used


/001

143.52.106.32/27

1st Floor




/010

143.52.106.64/27

2nd Floor


/011

143.52.106.96/27

3rd Floor


/100

143.52.106.128/27

4th Floor


/101

143.52.106.160/27

not used


/110

143.52.106.192/27

not used


/111

143.52.106.224/27

Cannot be used

VLSM: Example

NIC Number

143.52.0.0


Hosts on the 4th floor in the Harmer building

011/01/010./100/00000

143.52.106.128/27

The Floor


/00001

143.52.106.129 1st host


/00010

143.52.106.130

2nd host


/00011

143.52.106.131

3rd host





.



.



.


.



.



.


.



.



.





/11111

143.52.106.159

Broadcast

Interconnection Requirements


In each town the buildings are interconnected in a mesh via
Frame Relay with dial
-
up connections for back up.


Each building needs 3(buildings) x 2 = 6 addresses


Potentially 7( buildings) x 2 = 14 addresses (if we have 8 buildings)


Each town needs 4(buildings) x 6 = 24 addresses


Potentially 8(buildings) x 14 =
112

addresses (if we have 8 buildings)


Each town has a central site, located within one of the buildings,
which connects to the other towns via Frame_Relay with dial
-
up
connections for back up


Each town needs at least 2(towns) x 2 = 4 addresses for serial
connections


Each country needs a total of 3(towns) x 4 =
12

addresses for serial
connections


Each town is connected to the rest of the company via an ISP,
using VPN technology. Some resilience is also needed


Each country needs at least 3 x 2 =
6

addresses

Total addresses per country = 112 + 12 + 6 =
130




Summary of requirements


Within each town we need:


112 addresses between buildings (64 networks)


For each country we need:


12 addresses for connections between towns (6 networks)


6 addresses for connections between a town and the ISP (3
networks)


For each country we can use a different subnet of the ones
available in the country subnets



VLSM: Example

NIC Number

143.52.0.0


Country
: 000

143.52.0.0/19


Ireland


001

143.52.32.0/19

Scotland


010


143.52.64.0/19

Wales


011


143.52.96.0/19

England


100


143.52.128.0/19
Ireland connections



101


143.52.160.0/19

Scotland connections



110


143.52.192.0/19

Wales connections



111


143.52.224.0/19

England connections


Example: England

143.52.224.0/19
can be further subnetted for serial connections



13 bits to play with


Each serial connection will have a subnet mask of /30


We have 2
11

possible networks


143.52.111/00000.
000
000 143.52.224.0/30 Cannot be used




00000.000001 143.52.224.4/30




00000.000010 143.52.224.8/30




00000.000011 143.52.224.12/30




00000.000100 143.52.224.16/30




00000.000101 143.52.224.20/30




.

.


.


.




.

.


. .




11111.111110 143.52.255.248/30




11111.
111
111 143.52.255.252/30 Cannot be used

Recap


Issues in Network design


Designing an Addressing scheme with IP4


Subnetting


CIDR


Classful vs Classless


VLSM


Examples


References:

Understanding IP Addressing: Everything You Ever


Wanted To Know. 3COM whitepaper


http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf