IP Addressing 1

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24 Οκτ 2013 (πριν από 3 χρόνια και 5 μήνες)

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IP Addressing 1


IP addresses uniquely identify a host or
router on the Internet


It is a 32
-
bit address (4 octets)


Has two main fields: NetId and HostId


The MSBs of the NetId identify the IP class


There are five IP classes: A, B, C, D, E

IP Addressing 2


Decimal dotted notation Ex. 124.10.21.3


Classes:


A: MSB =0, from 1 to 126 first octet


B: MSBs =10, from 128 to 191 first octet


C: MSBs = 110 from 192 to 223 first octet


D: MSBs =1110 from 224 to 239 first octet


E: MSBs = 1111 from 240 to 255 first octet

IP Addressing 3


Classes A, B, C are used for unicasting


Class D is used for multicasting. There is no
NetId nor HostId, all bits are used for the
different multicasting addresses.


Class E is reserved by the Internet for
special use


IP Addressing 4


Number of Networks and Hosts per class


Class

Networks



Hosts


A


2
7
-

2 = 126

2
24
-
2 = 16777214


B


2
14

= 16384

2
16

-
2 = 65534


C


2
21

= 2097152

2
8

-

2 = 254


D


N.A.



N.A.


E


N.A.



N.A

IP Addressing 5


Multihomed devices:


Can be computers that are connected to
different networks, with different IP numbers
for each


Routers, each of its interfaces must have an
assigned IP number

IP Addressing 6


Special addresses (Can not be assigned to
hosts)


Network Address

Has Host ID all zeros


Direct Broadcast

Has host ID all 1s


Limited Broadcast

Has Net and Host ID all 1s


This host on this net


All zeros


Specific host/this net NetId= 0s, HostID specific


Loopback address


NetId = 127, HostID any

IP Addressing 7


Limited Broadcasts will be blocked by a
router and confined to the local network


This host on this network is an IP address used
by a host at bootup, when it does not know its
IP address and sends it as source address to
the server (all zeros)


Specific host on this network is an IP address
used by a host to communicate with another


IP Addressing 8


Specific host on this network ( NetID = 0,
HostID = specific )


Allows a host to communicate with another host
on the same network


Used only in the destination field of an IP packet


Loopback address (127 in first octet)


The packet with this destination address will not
leave the machine. Used to test the machine

IP Addressing 9


Multicast addresses


Are class D


Can be used only as destination address


Can be local or global level



Local means a group of users in one network


Global means a group of users belonging to different
networks)


A system (host) can have several multicast addresses in
addition to its own unicast address

IP Addressing 10


Assigned Multicast Addresses (by internet)



Address

Group


224.0.0.0

Reserved


224.0.0.1

All SYSTEMS on this SUBNET


224.0.0.2

All ROUTERS on this SUBNET


224.0.0.4

DVMRP routers


224.0.0.5

OSPFIGP All ROUTERS


224.0.0.6

OSPFIGP Designated ROUTERS

IP Addressing 11


Assigned Multicast Addresses (by internet)



Address

Group


224.0.0.7

ST Routers


224.0.0.8

ST Hosts


224.0.0.9

RIP2 Routers


224.0.0.10

IGRP Routers


224.0.0.11

Mobile
-
Agents


IP Addressing 12


Assigned Multicast Addresses (by internet)



Address

Group


224.0.1.X

For Conferencing/Teleconferencing


Private Networks


For organizations that want to use TCP/IP but
do not need internet access.


Although they could pick any set of IP
addresses, this could cause some confusion

IP Addressing 13


A block of IP addresses on the three classes
have been reserved for this purpose



CLASS

NETIDs


TOTAL Nets


A

10.0.0

.




1


B

172.16 to 172.31



16


C

192.168.0 to 192.168.255

256

IP Addressing
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Subnetting






14


Subnetting is the process of dividing a
network into several smaller subnetworks


To the Internet, the organization that subnetted
its network still has one network address.


The router at the organization will direct the
traffic to the different subnets, based on the
subnet masks placed at each subnet.

IP Addressing
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Subnetting






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Each of the three primary IP address classes
has an associated
default subnet mask


The subnet mask is a 32
-
bit number (also four
decimal dotted notation) in which the bits that
correspond to the IP network address are
made equal to one (1) and the bits that
correspond to the hosts (in the IP address) are
made equal to zero.

IP Addressing
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Subnetting






16


The default subnet masks for classes A, B, C
are:


Class A: 255.0.0.0


Class B: 255.255.0.0


Class C: 255.255.255.0


A network using the default subnet mask
is
not subnetted.

IP Addressing
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Subnetting






17


Depending on the available routers and
routing protocols, you can implement one of
two types of subnetting:


Constant Length subnet masks (CLSM)


Variable Length subnet mask (VLSM)


When you subnet, you are borrowing bits
from the HostId field and assign them to the
NetId field; that way you have more bits to
encode subnets Ids
but

less bits for hosts

IP Addressing
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Subnetting






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Example of CLSM: IP address= 130.25.0.0


This is a class B IP network address and has an
associated default subnet mask of 255.255.0.0


This two pieces of information could have been
obtained using another IP addressing notation
called
slashed IP
. In this example the IP network
address would be 130.25.0.0 /16 (slash 16) which
means that 16 bits are used for the associated
NetId

field of the subnet mask.

IP Addressing
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Subnetting






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Example of CLSM: IP address= 130.25.0.0


If we want to subnet it, and we use for example a
two bit subnet field
, then two bits will be
borrowed from the HostId field, of the default
subnet mask, and will be used to identify up to
four possible subnets: using the bit combinations:
00, 01, 10, 11.


The subnet mask will become: 255.255.192.0


IP Addressing
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Subnetting






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Example of CLSM: IP address= 130.25.0.0


Working it out in binary for : 255.255.192.0


1111 1111.1111 1111.
11
00 0000.0000 0000


The group of bits=1 not bolded, correspond to
what is called the
network prefix.


The two bits=1 that are bolded are the subnet Id.


The collection of all the bits=1 is the
extended
network prefix.

IP Addressing
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Subnetting






21


Using two bits for subnetId (b=2) allows us to
subnet into 2
b

=2
2 =

4 subnets. But since the
all zeros and all ones combinations are not
allowed, the
usable subnets are

2
b



2 = 2


From a routing perspective, subnetting can be
used to create network regions where
machines would be on the same physical
network(segment) and thus communicate
using their MAC addresses

IP Addressing
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Subnetting






22


When a computer in one subnet wishes to
communicate with a computer on another
subnet, traffic must be forwarded from the
sender to a
nearby IP gateway

(router).


The Gateway will send its message on its way
to the destination, which could be local to the
router (another port) or to another router that
is closer to the destination host.

IP Addressing
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Subnetting






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Exercise of CLSM subnetting:


Net Inc. wants 12 subnets for its class C IP
network address 200.10.10.0. No subnet will
require more than 10 hosts addresses


Solution:


1. Add two to the number of subnets needed. In this
case 12 + 2 = 14.


Find the power of two closest to that number but larger
to it. In this case 16

IP Addressing
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Subnetting






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Solution:


3. Which exponent must be used in base two to obtain
the number of subnets calculated before? 2
n

= 16 ?
Answer, n = 4


4. Thus you will borrow 4 bits from the associated
HostId bits of default subnet mask and use them for the
associated NetId bits of the subnet mask to be used


In binary : 1111 1111.1111 1111.1111 1111.
1111

0000


In dotted decimal: 255.255.255.240


IP Addressing
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Subnetting






25


Variable Length Subnet Mask (VLSM)


If all segments(subnets) must support roughly the
same number of hosts (+/
-

20%), CLSM makes
the most sense; but if some subnets require a large
number of hosts and others do not, CLSM will
waste IP address space. In these cases VLSM is
used.


Basically with VLSM, the larger subnets are
subnetted again, using a second subnet mask.

IP Addressing
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Subnetting






26


Variable Length Subnet Mask (VLSM)


To implement VLSM, you must have:


1. Routing protocols that carry the extended
network prefix information with each route
advertised. For example: RIP
-
2, and OSPF.


2. The routers must implement a “longest match”
algorithm, whereby, the router will select the route
with the longest extended network prefix (the
most specific network), when several routes
available

IP Addressing
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Subnetting






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Variable Length Subnet Mask (VLSM)


3. The address assigned must mirror the physical
topology of the network. This reduces the amount
of routing information by taking the set of
addresses assigned to a particular region of the
topology and aggregating them into a single
routing advertisement for the entire set.


This issue is important for routing purposes and is
called CIDR (Classless Interdomain Routing)