CCNA Exploration Semester 1-Chapter 5 ppt

Network layer topicsrouted protocol Routing – sending packets the right way... Layer 3 protocolA layer 3 protocol such as IP version 4 must: networks and individual hosts Encapsulate a

OSI network layer

CCNA Exploration Semester 1

CCNA Exploration Semester 1

Chapter 5

OSI network layer

TCP, UDP

IP

Ethernet, WAN technologies

HTTP, FTP, TFTP, SMTP etc

Segment

Packet

Frame Bits Data stream

Network layer topics

routed protocol

Routing – sending packets the right way

Purpose of layer 3

destination, then route it.

Layer 3 protocol

A layer 3 protocol such as IP version 4 must:

networks and individual hosts

Encapsulate a segment from layer 4 into a

Encapsulate a segment from layer 4 into a

packet and include addresses

Direct the packet across one or many

networks to the destination host

give the segment to layer 4

Network Layer – Communication from

Host to Host

Layer 3 uses four basic processes:

1 Addressing

2 Encapsulation

3 Routing: Intermediary devices that connect the networks

are called routers The role of the router is to select

are called routers The role of the router is to select

paths for and direct packets toward their destination

This process is known as routing.

4 Decapsulation

Network layer protocols

common

to replace version 4 eventually

to replace version 4 eventually

(CLNS/DECNet)

The roles of IPv4 protocol

IPv4 is currently the most widely-used version of IP

with low overhead It provides only the functions that are necessary to deliver a packet from a source to a destination over an interconnected system of

networks.

the flow of packets These functions are performed

by other protocols in other layers.

IP characteristics

does only what it needs to do

with destination before sending packet

with destination before sending packet

delivery, no checking or resending

know maximum packet size

IP does not notify the destination host.

Which layer 4 protocol on the sending host will establish a

connection?

TCP: A connection-oriented protocol

Which layer 4 protocol on the sending host will not establish a connection?

Best Effort Service (unreliable)

Layer 3 (IP)

Speed over reliability

Unreliable: Does not have the capability or responsibility to

manage, and recover from, undelivered or corrupt packets.

Who does?

TCP at the end-to-end hosts

Media Independent

Responsibility of the OSI Data Link layer to take an IP packet and prepare it for transmission over the communications medium

Transport of IP packets is not limited to any particular medium.

May need to fragment the packet if it is too many bits (later).

Network layer encapsulation

Segment from transport layer

Packet header added to make

Packet header added to make

IP packet

Sent to data link layer for

further encapsulation into

frame

IPv4 packet header fields

IP address of source

host, needed so reply

can be sent.

IP address of destination host, needed so routers can find route.

IPv4 packet header fields

Reduced by 1 at each

router Packet dropped if

it goes to 0.

TCP or UDP used in Transport layer.

Protocol field enables the Network layer to pass

the data to the appropriate upper-layer protocol.

01 ICMP

06 TCP

17 UDP

17 UDP

UNIX: 255

Linux: 64 or 255 depending upon vendor and version

Microsoft Windows 95: 32

Microsoft Vista: 128

IPv4 packet header fields

Priority for QoS E.g

voice data has higher

priority than e-mail.

For checking if header has been corrupted.

IPv4 packet header fields

Shows if packet has

been fragmented or

must not be fragmented.

If router has to split a packet, this gives order for putting pieces together.

IP Fragmentation

Data = 1480 bytes IP

Data = 500 IP

Data = 480 IP

A router may have to fragment a packet when forwarding it from one

medium to another medium that has a smaller MTU.

Data = 520

L2 L2 go over my serial link all This packet is too big to

at once I need to break it into smaller fragments

Data = 520

Data = 500

Network Layer Fields

• 0: This is the last fragment

• 1: More fragment follow this fragment

IPv4 packet header fields

length.

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

How to split the network

resources do people use? How much

bandwidth do they use?

bandwidth do they use?

departments in a company, security

requirements

Broadcast domain ? - Performance

A broadcast is a message sent from one host to all other hosts on the network.

Managing the size of broadcast domains by dividing a network into subnets or replace switch by router

Hierarchical Addressing

devices communicating across networks of

networks

IPv4 hierarchical address

32 bits in four 8-bit octets, written in decimal

Here network part (prefix) is 24 bits /24

Message to same network

Message to different network

Default gateway

and a default gateway

router port on the same network as the PC

router port on the same network as the PC

other networks

has a different IP address

packet header and decides what to do with

the packet next

A Quick Look at Routing

A Quick Look at Routing

Host Routing Table

netstat –r

or

Hosts also have a local routing table.

Usually only contains:

Its own network address (directly connected network)

Default gateway IP address

route print

Network 192.168.2.0/24 Network 192.168.1.0/24

Routing – First Look

Routers know about:

Directly connected networks (C):

Network addresses of its interfaces

When a router is configured with the IP address/mask on an interface the router knows that it has an interface which is part of that network.

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24 Network 192.168.1.0/24

Routing – First Look

Routers learn about remote networks using:

Static routes

Dynamic Routing Protocol (R = RIP)

Routes in a routing table have three main features:

Destination network

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Routing table and forwarding

a list of known networks and the best way to get there – outgoing port and address of next-hop router

hop router

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

Directly connected

The networks of the router’s own interfaces go into the routing table.

Other networks

by an administrator (static routes)

using a routing protocol (dynamic routes)

using a routing protocol (dynamic routes)

unknown networks go on this route instead of being dropped

Routing table entries

Directly connected shown by C

Routing table entries

Static, configured by administrator, shown by S

Routing table entries

Default, configured by administrator, shown by S*

Routing table entries

Learned from another router using RIP routing protocol, shown by R

Router has a route

Routing protocols

in their routing tables.

A routing protocol is the set of rules they use to

swap information.

Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway

Protocol (BGP) and Enhanced IGRP (EIGRP

Routed versus Routing

Routed protocol: used at the network layer

that transfer data from one host to another across a router

Routing protocols: allow routers to choose

Routing protocols: allow routers to choose

the best path for data from source to

destination

Internetwork Packet Exchange (IPX);

DECnet, AppleTalk, Banyan VINES, and

Xerox Network Systems (XNS)

Default route

default route is a route that will match all destination networks In IPv4 networks, the address 0.0.0.0 is used for this purpose.

which there is no entry in the routing table for the

destination network.

does not match a more specific route in the routing table are forwarded to the nexthop router associated with the default route.

Static routes Dynamic routes

administrator

different for each router

Labs

The End The End