Download Network Layer - SI-35-02

Network Layer
Overview
• The network layer is responsible for navigating
the data through the network.
• The function of the network layer is to find the
best path through the network.
• The network layer's addressing scheme is used by
devices to determine the destination of data as it
moves through the network.
• In this chapter, you will learn about the router’s
use and operations in performing the key
internetworking function of the Open System
Interconnection (OSI) reference model’s network
layer, Layer 3.
Overview
• In addition, you will learn about IP addressing
and the three classes of networks in IP addressing
schemes.
• You also will learn that some IP addresses have
been set aside by the American Registry for
Internet Numbers (ARIN) and cannot be assigned
to any network.
• Finally, you will learn about subnetworks and
subnet masks and their IP addressing schemes.
Importance of a Network Layer :
Identifiers
• The network layer is responsible for moving data through a
set of networks (internetwork).
• The network layer's addressing scheme is used by devices to
determine the destination of data as it moves through the
networks.
Importance of a Network Layer :
Identifiers
• Protocols that have no network layer can only be
used on small internal networks.
• These protocols usually use only a name (i.e.
MAC address) to identify the computer on a
network.
• The problem with this approach is that, as the
network grows in size.
• It becomes increasingly difficult to organize all
the names, such as making sure that two
computers aren't using the same name.
Importance of a Network Layer :
Identifiers
• Protocols that support the network layer use a
hierarchical addressing scheme that allows for
unique addresses across network boundaries,
along with a method for finding a path for
data to travel between networks.
• While MAC addresses use a flat addressing
scheme that makes it difficult to locate
devices on other networks.
Importance of a Network Layer :
Identifiers
• Hierarchical addressing schemes enable information to
traverse an internetwork, along with a method to find
the destination in an efficient fashion.
• The telephone network is an example of the use of
hierarchical addressing.
• The telephone system uses an area code that
designates a geographical area for the call's first stop
(hop).
• The next three digits represent the local exchange
(second hop).
• The final digits represent the individual destination
telephone (which is, or course, the final hop).
Importance of a Network Layer :
Identifiers
• Network devices need an addressing scheme
that allows them to forward data packets
through the internetwork (a set of networks
composed of multiple segments using the
same type of addressing).
• There are several network layer protocols with
different addressing schemes that allow
devices to forward data throughout an
internetwork.
Importance of a Network Layer :
Segmentation and autonomous systems
• There are two primary reasons why multiple
networks are necessary - the growth in size of
each network and the growth in the number of
networks.
• When a LAN, MAN, or WAN grows, it may
become necessary or desirable for network traffic
control to break it up into smaller pieces called
network segments (or just segments).
• This results in the network becoming a group of
networks, each requiring a separate address.
Importance of a Network Layer :
Segmentation and autonomous systems
• There are already a vast number of networks in
existence; separate computer networks are common in
offices, schools, companies, businesses, and countries
• It is convenient to have these separate networks (or
autonomous systems, if each is managed by a single
administration) communicate with each other over the
Internet.
• However, they must do it with sensible addressing
schemes and appropriate internetworking devices.
• If not, the network traffic flow would become severely
clogged, and neither the local networks, nor the
Internet, would function.
Importance of a Network Layer : Segmentation
and autonomous systems
• An analogy that might help you understand the need for network
segmentation is to imagine a highway system and the number of vehicles
that use it.
• As the population in the areas surrounding the highways increases, the
roads become burdened with too many vehicles.
• Networks operate much in the same way. As networks grow, the amount
of traffic grows.
• One solution might be to increase the bandwidth, much the same as
increasing the speed limits of, or adding lanes to, the highways.
• Another solution might be to use devices that segment the network and
control the flow of traffic, the same way a highway would use devices such
as stoplights to control the movement of traffic.
Importance of a Network Layer :
Segmentation and autonomous systems
Importance of a Network Layer :
Communication between separate networks
• The Internet is a collection of network
segments that are tied together to facilitate
the sharing of information.
• Once again, a good analogy would be the
example of the highway system with the large
multiple lanes that have been constructed to
interconnect many geographical regions.
Importance of a Network Layer :
Communication between separate networks
• Networks operate in much the same manner, with companies
known as Internet service providers (ISPs) offering services
that tie together multiple network segments.
Importance of a Network Layer :
Layer 3 network devices
• Routers are internetworking devices which operate at OSI
layer 3 (the network layer).
• They tie together, or interconnect, network segments or
entire networks.
• They pass data packets between networks based on Layer 3
information
Importance of a Network Layer :
Layer 3 network devices
• Routers make logical decisions regarding the best
path for the delivery of data on an internetwork
and then direct packets to the appropriate output
port and segment.
• Routers take packets from LAN devices (e.g.
workstations) and, based on Layer 3 information,
forward them through the network.
• In fact, routing is sometimes referred to as Layer
3 switching
Path Determination
• Path determination occurs at Layer 3 (network layer).
• It enables a router to evaluate the available paths to
a destination, and to establish the preferred handling
of a packet.
• Routing services use network topology information
when evaluating network paths.
• Path determination is the process that the router
uses to choose the next hop in the path for the
packet to travel to its destination.
• This process is also called routing the packet.
Path Determination
Path Determination
• Path determination for a packet can be
compared to a person driving a car from one
side of a city to the other.
• The driver has a map that shows the streets
that he/she needs to take to get to the
destination.
• The drive from one intersection to another is a
hop. Similarly, a router uses a map that shows
the available paths to a destination.
Path Determination
• Routers can also make their decisions based
on the traffic density and the speed of the link
(bandwidth)
• Just as a driver may choose a faster path (a
highway) or use less crowded back streets.
Path Determination : Network layer addressing
• The network address helps the router identify
a path within the network cloud.
• The router uses the network address to
identify the destination network of a packet
within an internetwork.
Path Determination : Network layer
addressing
• For some network layer protocols, a network
administrator assigns network addresses according to
some predetermined internetwork addressing plan.
• For other network layer protocols, assigning addresses
is partially or completely dynamic/automatic.
• In addition to the network address, network protocols
use some form of host, or node, address.
• The graphic shows three devices in Network 1 (two
workstations and a router), each with its own unique
host address.
• (it also shows that the router is connected to two other
networks - Networks 2 & 3).
Path Determination : Network layer
addressing
Path Determination : Network layer
addressing
• Addressing occurs at the network layer.
• Earlier analogies of a network address include the
first portions (area code and first three digits) of a
telephone number.
• The remaining (last four) digits of a phone
number tell the phone company equipment
which specific phone to ring.
• This is similar to the function of the host portion
of an address.
• The host portion tells the router to which specific
device it should deliver a packet.
Path Determination : Network layer
addressing
• Without network layer addressing, routing can not take
place.
• Routers require network addresses to ensure proper
delivery of packets.
• Without some hierarchical addressing structure,
packets would not be able to travel across an
internetwork.
• In a similar way, without some hierarchical structure to
telephone
numbers,
postal
addresses,
or
transportation systems, there would not be a smooth
delivery of the goods and services.
Path Determination : Layer 3 and
computer mobility
• A MAC address can be compared to your name and the
network address to your mailing address.
• For example, if you were to move to another town,
your name would remain unchanged, but your mailing
address would indicate your new location.
• Network devices (routers as well as individual
computers) have both a MAC address and a protocol
(network layer) address.
• When you physically move a computer to a different
network, the computer maintains the same MAC
address, but you must assign it a new network address.
Path Determination : Comparing flat and
hierarchical addressing
• The function of the network layer is to find the
best path through the network.
• To accomplish this, it uses two addressing
methods - flat addressing and hierarchical
addressing.
• A flat addressing scheme assigns a device the
next available address.
• There is no thought given to the structure of
the addressing scheme.
Path Determination : Comparing flat
and hierarchical addressing
• An example of a flat addressing scheme would
be military identification numbering system,
or a birth identification numbering system.
• MAC addresses function in the same manner.
• A vendor is given a block of addresses; the
first half of each address is for the vendor's
code, the rest of the MAC address is a number
that has been sequentially assigned.
Path Determination : Comparing flat
and hierarchical addressing
• The postal system ZIP codes are a good example of
hierarchical addressing.
• In the ZIP code system the address is determined by
the location of the building, not by a randomly
assigned number.
• The addressing scheme that you will use throughout
this course is Internet Protocol (IP) addressing.
• IP addresses have a specific structure and are not
randomly assigned.
Path Determination : Comparing flat
and hierarchical addressing
IP Addresses within the IP Header :
Network layer datagrams
• The Internet Protocol (IP) is the most popular
implementation of a hierarchical network
addressing scheme.
• IP is the network protocol the Internet uses.
• As information flows down the layers of the
OSI model, the data is encapsulated at each
layer.
• At the network layer, the data is encapsulated
within packets (also known as datagrams).
IP Addresses within the IP Header :
Network layer datagrams
• IP determines the form of the IP packet header
(which includes addressing and other control
information), but does not concern itself with the
actual data -- it accepts whatever is passed down
from the higher layers.
IP Addresses within the IP Header :
Network layer datagrams
IP Addresses within the IP Header :
Network Layer Fields
IP Addresses within the IP Header :
Network Layer Fields
• The Layer 3 packet/datagram becomes the Layer 2
data, which is then encapsulated into frames (as
previously discussed).
• Similarly, the IP packet consists of the data from
upper layers plus an IP header, which consists of:
– version - indicates the version of IP currently used (4 bits)
– IP header length (HLEN) - indicates the datagram header
length in 32 bit words (4 bits)
IP Addresses within the IP Header :
Network Layer Fields
– type-of-service - specifies the level of importance that
has been assigned by a particular upper-layer protocol
(8 bits)
– total length - specifies the length of the entire IP
packet, including data and header, in bytes (16 bits)
– identification - contains an integer that identifies the
current datagram (16 bits)
– flags - a 3-bit field in which the 2 low-order bits
control fragmentation – one bit specifying whether
the packet can be fragmented, and the second
whether the packet is the last fragment in a series of
fragmented packets (3 bits)
IP Addresses within the IP Header :
Network Layer Fields
– fragment offset - the field that is used to help piece
together datagram fragments (16 bits)
– time-to-live - maintains a counter that gradually decreases,
by increments, to zero, at which point the datagram is
discarded, keeping the packets from looping endlessly (8
bits)
– protocol - indicates which upper-layer protocol receives
incoming packets after IP processing has been completed
(8 bits)
– header checksum - helps ensure IP header integrity (16
bits)
IP Addresses within the IP Header :
Network Layer Fields
– source address - specifies the sending node (32
bits)
– destination address - specifies the receiving node
(32 bits) options - allows IP to support various
options, such as security (variable length)
– data - contains upper-layer information (variable
length, maximum 64 Kb)
– padding - extra zeros are added to this field to
ensure that the IP header is always a multiple of
32 bits
IP Addresses within the IP Header: IP header
source and destination fields
• The IP address contains the information that is
necessary to route a packet through the
network.
• Each source and destination address field
contains a 32 bit address.
• The source address field contains the IP
address of the device that sends the packet.
• The destination field contains the IP address
of the device that receives the packet.
IP Addresses within the IP Header: IP
header source and destination fields
IP Addresses within the IP Header :
IP address as a 32-bit binary number
• An IP address is represented by a 32 bit binary number.
• As a quick review, remember that each binary digit
can be only 0 or 1.
• In a binary number, the value of the right-most bit (also
called the least significant bit) is either 0 or 1.
• The corresponding decimal value of each bit doubles as
you move left in the binary number.
• So the decimal value of the 2nd bit from the right is
either 0 or 2. The third bit is either 0 or 4, the fourth bit
0 or 8, etc ...
IP Addresses within the IP Header :
IP address as a 32-bit binary number
IP Addresses within the IP Header :
IP address as a 32-bit binary number
• IP addresses are expressed as dotted-decimal
numbers - we break up the 32 bits of the address
into four octets (an octet is a group of 8 bits).
• The maximum decimal value of each octet is 255.
• The largest 8 bit binary number is 11111111.
• Those bits, from left to right, have decimal values
of 128, 64, 32, 16, 8, 4, 2, and 1. Added together,
they total 255.
IP Addresses within the IP Header:
IP address component fields
• The network number of an IP address identifies
the network to which a device is attached.
• The host portion of an IP address identifies the
specific device on that network.
• Because IP addresses consist of four octets
separated by dots, one, two, or three of these
octets may be used to identify the network
number.
• Similarly, up to three of these octets may be used
to identify the host portion of an IP address.