5network layer

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17-Feb-11 S Ward Abingdon and Witney College CCNA Exploration Semester 1

OSI network layer

CCNA Exploration Semester 1

Chapter 5


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2

Tujuan Lapis Network

Menjamin Komunikasi end to end,

mendapatkan data dari sumber ke tujuan,

dan me-route-kannya: Addressing/Pengalamatan

Encapsulation

Routing

Decapsulation



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OSI network layer

OSI model layer3

TCP/IP model Internet layer

Application

Presentation

Session

Transport

Network

Data link

Physical

Application

Transport

Internet

Network Access

TCP, UDP

IP

Ethernet,

WAN

technologies

HTTP, FTP,

TFTP, SMTP

etc

Segment

Packet

Frame

Bits

Data

stream


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5

Network Layer Protocol

Protocols implemented at the Network layer

that carry user data include:

Internet Protocol version 4 (IPv4) Internet Protocol version 6 (IPv6)

Novell Internetwork Packet Exchange (IPX)

AppleTalk

Connectionless Network Service (CLNS/DECNet)



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6

Protocol IPv4

Characteristics IPv4

Connectionless - No connection is established

before sending data packets.

Best Effort (unreliable) - No overhead is used to

guarantee packet delivery.

Media Independent - Operates independently of

the medium carrying the data.



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IPv4 - Connectionless



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IPv4 Best effort (Unreliable)

IP tidak mempunyai kemempuan untuk

memanage dan recovery packet yang tak

terkirim, hilang, atau rusak.



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IPv4 - Media Independent



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Network layer topics

IP version 4 the most common layer3

routed protocol

Dividing hosts into groups why and how Routing sending packets the right way

Routing how routers learn routes

IP addressing in chapter 6


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Network layer encapsulation

Segment from transport layer

Packet header added to make

IP packet

Sent to data link layer for

further encapsulation into

frame


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IPv4 packet header fieldsIP address of source

host, needed so reply

can be sent.

IP address of destination

host, needed so routers

can find route.


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IPv4 packet header fieldsReduced by 1 at each

router. Packet dropped if

it goes to 0.

TCP or UDP used in

Transport layer.


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IPv4 packet header fieldsPriority for QoS. E.g.

voice data has higher

priority than e-mail.

For checking if header has

been corrupted.


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IPv4 packet header fieldsShows if packet has

been fragmented or

must not be fragmented.

If router has to split a

packet, this gives order for

putting pieces together.


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IPv4 packet header fieldsVersion 4. Length of whole packet.Header

length.


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Splitting up networks

Fully switched network, each device has its own

bandwidth. You could have hundreds of computers.

Why split it up?

Too large to manage efficiently

Too much broadcast traffic - congestion

Too many addresses for switches to remember

Lack of security


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How to split the network

Geographically different sites


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How to split the network

(contd)

Ownership different companies or departments in a

company, security requirements



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Use a router

Limits broadcasts

Can provide security

Addressing scheme

based on networks -

hierarchical


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IPv4 hierarchical address

32 bits in four8-bit octets, written in decimal

Network part then host part

Here network part (prefix) is 24 bits /24

Length of network part can vary.


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Message to same network


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Message to different network


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Default gateway

Each PC is configured with an IP address

and a default gateway.

The default gateway is the IP address of arouter port on the same network as the PC.

It is the routers job to handle messages to

other networks.

Each router port is on a different network and

has a different IP address.


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Hops

A packet may pass through many routers on

its journey.

The trip from one router to the next is called ahop and the next router is called the next hop

router.

Each router looks at the IP address in the

packet header and decides what to do withthe packet next.


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Routing table and forwarding

Each router has a routing table. This contains

a list of known networks and the best way to

get there outgoing port and address of next-

hop router.

The router looks at the IP address of a packet.

It decides which network this address is on.

If it knows the network it forwards the packet.If it does not know the network it drops the

packet.


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Directly connected

The networks of the routers own interfaces go into the

routing table.


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Other networks

Routes to other networks can be configured

by an administrator (static routes)

Or they can be learned from another routerusing a routing protocol (dynamic routes)

A router can have a default route. Packets for

unknown networks go on this route instead of

being dropped.


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Routing table entries

Directly connected shown by C


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Static, configured by administrator,

shown by S


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Learned from another router using RIP

routing protocol, shown by R


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Router has a route


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Routing protocols

Routers learn routes from each other and put

them in their routing tables.

A routing protocol is the set of rules they useto swap information.

These routes are dynamic routes


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Static routes Dynamic routes

Entered by

administrator

Time consuming,

different for each router

Must be updated if

routes change

Little processing

No bandwidth used

Gives nothing away

Learned from other

routers

Start the protocol then

it runs by itself

Automatically updates

when routes change

More processing

Uses bandwidth

Gives away information


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