Planning the Addressing Structure

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Oct 24, 2013 (3 years and 7 months ago)

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Planning the
Addressing Structure

Working at a Small
-
to
-
Medium Business or ISP


Chapter 4


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Objectives


Describe how IP Addressing is implemented in the
LAN


Subnet a given network to allow for efficient use of IP
address space


Explain how Network Address Translation (NAT) and
Port Address Translation (PAT) are used in a
network

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Implementation of IP Addressing in the LAN


The purpose of an IP address


IP address hierarchical structure


The classes of IP addresses

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Review of IP addressing


IP addressing

is the method used to identify hosts
and network devices.


In order to send and receive messages on an IP
network, every network host must be assigned a
unique 32 bit IP address.


Because large binary numbers are difficult for humans
to read and understand, IP addresses are usually
displayed in dotted
-
decimal notation.


IP addresses are hierarchical therefore for a network,
this means that part of the 32
-
bit number identifies the
network (parent) while the rest of the bits identify the
host (child).


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Review of IP addressing


As the number of hosts connected to the Internet
continues to grow, and the IP addressing scheme has
to be adapted to cope with this growth.


In order to cope with the demand, more unique
network numbers were required to create more
possible network designations, the 32
-
bit address
space was organized into five classes.


Three of these classes, A, B, and C, provide
addresses that can be assigned to individual hosts or
networks. The other two classes, D and E, are
reserved for multicast and experimental use
respectively.


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Review of IP addressing


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Implementation of IP Addressing in the LAN


Classful subnetting including how subnet masks are
used in calculations for addressing and routing, and IP
address notation for subnet masks

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Classful Subnetting


RFC 917,
Internet Subnets
, defines the subnet mask as
the method routers use to isolate a subnet from an IP
address.


When a router receives a packet it uses the destination IP
address in the packet and the subnet masks associated
with the routes in its routing table to determine the
appropriate path on which to forward the packet.


The router reads the subnet mask from left to right, bit for
bit.


If a bit in the subnet mask is set to 1, it indicates that
the value in that position is part of the network ID.


A 0 in the subnet mask indicates that the value in that
position is part of the host ID.

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Classful Subnetting


The two
-
level hierarchy of classed addressing included
a network ID and a host ID.


In classful subnetting, the network ID is left alone, and
the host ID is divided into a subnet ID and a new host
ID.


For example, a Class B network has a 16
-
bit default
subnet mask of 11111111 11111111 00000000
00000000, or 255.255.0.0. That leaves 16
-
bits for the
host ID.


To divide a class B into multiple networks is to use four
of the host bits as a subnet ID. There is now a 20
-
bit
subnet mask of 255.255.240.0, and only 12
-
bits remain
for the host ID.

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Implementation of IP Addressing in the LAN


Identifing the number of subnet bits required for a given
network implementation

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Classful Subnetting


Subdividing a network adds a level to the network
hierarchy. Now there are three levels:
a network, a
subnetwork, and a host
. How are these three levels
identified?


In classful addressing, the number of network bits is
fixed. There are 8 bits that designate a Class A
network, 16 bits for a Class B, and 24 for a Class C.
That leaves the host bits as the only part of the IP
address with any flexibility to modify.


There are two considerations when planning subnets:
the number of hosts on each network, and the
number of individual local networks needed
.

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Classless Subnetting


CIDR


VLSM

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Classless Subnetting


Partitioning the host ID this way always results in a fixed
number of subnets and a fixed number of hosts per
subnet.


In a situation where an organization has a Class B
network with four subnets, thousands of IP addresses
can be wasted if some of the subnets have only a few
hosts in them.


Therefore to use IP addresses more efficiently,
Classless Inter
-
Domain Routing (CIDR)

was created.


With CIDR, there are no more network classes. CIDR
uses
variable length subnet masks (VLSM)

for
subnetting.

The network ID no longer has to be on an octet
boundary.

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Classless Subnetting


Using CIDR addressing, sometimes referred to as
classless addressing, the number of bits that can make
up the network ID is not restricted by class.


Networks can be created that use the 192.168.0.0
address space with fewer than 24 bits indicating the
network number
.

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Creating Custom Subnet Masks


Communicating between subnets

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Custom Subnet Masks


The number of bits for a subnet ID that will be added to
the subnet mask depends on several factors.


For instance, in an organization assigned a Class C
address, what if there are multiple networks, one
network with 7 hosts, another with 60 hosts, and a third
with 34 hosts?


In classed subnetting, all subnets must be the same
size, which means that the minimum number of hosts
that each subnet must support is 60.


To support a minimum number of 60 hosts, at least 6
bits are required in the host ID, which leaves 2 bits for
the subnet identifier. Under these conditions, four
subnets can be created, each with 64 hosts.

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Custom Subnet Masks


If a Class C network is subnetted and 3 bits are taken
from the host ID to use for the subnet ID, there are 5
bits left for host addresses. Five host bits mean that
there can be 30 hosts per subnet, or 2^5
-

2.


The number of subnets is calculated in a similar
manner. If 3 bits are used for the subnet address, the
number of subnets is 2x2x2, or 2^3. By subnetting in
this manner, there are 8 subnets with 30 hosts each.


When determining how many hosts are needed in
each subnet, it is necessary to include the router
interface as well as the individual host devices. Each
router interface must have an IP address in the same
subnet as the host work attached to it.

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Custom Subnet Masks
-
The Subnetting


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Custom Subnet Masks
-
The Addressing


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Communicating between subnets


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Communicating between subnets


When a network is split into two subnets, there are
actually two separate networks.


Routers connect networks. In order for a device in
one subnet to communicate with a device in the
other, a router is required.


The configuration must ensure that interfaces on
routers that connect to each other are assigned IP
addresses in the same network or subnet, and that
clients are assigned default gateways that they can
reach.

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Implementation of IP Addressing in the LAN


The origin, purpose, and function of IPv6


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IPv6


CIDR and private IP addressing were developed to
provide a temporary solution to the problem of IP
address depletion. These methods, though useful, did
not create more IP addresses. IPv6 does that.


There were good reasons for IPv6 development.


More address space


Better address space management


Easier TCP/IP administration


Modernized routing capabilities


Improved support for multicasting, security, and
mobility

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IPv6


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Using Network Address Translation in a
Network


The purpose and function of network address
translation (NAT) and how it is implemented

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Network Address Translation (NAT)


Network Address Translation (NAT) allows a large
group of private users to access the Internet by sharing
a small pool of public IP addresses.


NAT was developed to save registered IP addresses.


NAT also provides security to PCs, servers, and
networking devices by withholding their actual IP host
addresses from direct Internet access (NAT helps
shield users of a private network against access from
the outside. )


The main advantage of NAT is IP address reuse, and
the sharing of globally unique IP addresses between
many hosts from a single LAN.


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Network Address Translation (NAT)

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Using Network Address Translation in a
Network


The terms used to describe how packets are
transported across a NAT configuration

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IP NAT Terms


The
inside local network

refers to any network
connected to a router interface that is part of the
privately addressed LAN. Hosts on inside networks
have their IP addresses translated before they are
transmitted to outside destinations.


The
outside global network

is any network attached to
the router that is external to the LAN and that does not
recognize the private addresses assigned to hosts on
the LAN.


An
inside local address

is the private IP address
configured on a host on an inside network. It is an
address that must be translated before it can travel
outside the local network addressing structure.

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IP NAT Terms


An
inside global address

is the IP address of an inside
host as it appears to the outside network. This is the
translated IP address.


The
outside local address

is the destination address of
the packet while it is on the local network. Usually this
address is the same as the outside global address.


An
outside global address

is the actual public IP
address of an external host. The address is allocated
from a globally routable address or network space
.

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Static and Dynamic NAT


One of the advantages of using NAT is that individual
hosts are not directly accessible from the public Internet.


What if one or more of the hosts within a network are
running services that need to be accessed from Internet
connected devices, as well as devices on the local
private LAN?


Therefore one way to provide access to a local host
from the Internet is to assign that device a
Static
address translation.


Static translations

ensure that an individual host
private IP address is always translated to the same
registered global IP address.

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Static and Dynamic NAT


It also ensures that no other local host will be
translated to the same registered address.


Dynamic NAT

occurs when a router is configured to
assign an IP address from an available pool of outside
global addresses to an inside private network device.


Dynamic NAT allows hosts assigned with private IP
addresses on a network, or intranet, to access a public
network, such as the Internet.


Static NAT allows hosts on the public network to access
selected hosts on a private network.


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Port
-
based Address Translation (PAT)


When an organization has a very small registered IP
address pool, or perhaps even just a single IP address,
it can still enable multiple users to simultaneously
access the public network with a mechanism called
NAT overload
, or
port address translation (PAT).


PAT translates multiple local addresses to a single
global IP address.


In PAT, the gateway translates the local source
address and port combination in the packet to a single
global IP address and a unique port number above
1024. Although each host is translated into the same
global IP address, the port number associated with the
conversation is unique.


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IP NAT Issues


The big issue with NAT is the additional work load
necessary to support IP address and port translations.


Some applications increase the work load of the router
because they embed an IP address as part of the
encapsulated data. The router must replace the source
IP addresses and port combinations that are contained
within the data, as well as the source addresses in the
IP header.


With all this activity taking place in a router because of
NAT, its implementation in a network requires good
network design, careful selection of equipment, accurate
configuration and regularly scheduled maintenance.

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Summary


IP addressing can be tailored to the needs of the
network design through the use of custom subnet
masks.


Classless subnetting gives classful IP addressing
schemes more flexibility through the use of variable
length subnet masks.


Network Address Translation (NAT) is a way to shield
private addresses from outside users.


Port Address Translation (PAT) translates multiple local
addresses to a single global IP address, maximizing the
use of both private and public IP addresses.

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