Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

Network Fundamentals

CH5: NETWORK LAYER

Abdelkhalik Elsaid Mosaabdu.elsaid@yahoo.com

abdelkhalik.staff.scuegypt.edu.eg

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

Introduction

• Transport Layer allows end-to-end transfer of application data.• Network Layer allows end-to-end device communication.

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Network Layer – Communication from host to host

• Layer 3 provides services to exchange the individual pieces of data over the network between identified end devices.

• To accomplish this end-to-end transport, Layer 3 uses four basic processes:1.Addressing2.Encapsulation3.Routing4.Decapsulation

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

Network Layer Protocols

• Routed Protocols: carry user data1. Internet Protocol version 4(IP v4), IP v6.2. Novell Internetwork Packet Exchange (IPX).3. AppleTalk.4. Connectionless Network Service (CLNS/DECNet).

• Routing Protocols: direct packet to destination1. RIP, BGP “application layer Protocols”2. IGRP, EIGRP3. OSPF4. IS-IS

• Other supporting protocols1. ICMP2. ARP

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IP v4 Characteristics

• IP v4 was designed as a protocol 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.

IP V4 Characteristics

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

Connectionless

• IP packets are sent without notifying the end host that they are coming.

• Connection-oriented protocols require that control data be exchanged to establish the connection.

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Best effort «Unreliable»

• Means that IP does not have the capability to manage, and recover from, undelivered or corrupt packets.

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Media Independent

• IPv4 and IPv6 operate independently of the media that carry the data at lower layers of the protocol stack.

• It is the responsibility of the OSI Data Link layer to take an IP packet and prepare it for transmission over the medium.

• MTU: the maximum sizeof a packet.

• The Data Link layer passesthe MTU upward to the Network layer.

• Intermediary device like a router usually fragments the packet

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IP v4 Packet Header

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IP v4 Packet Header

• Version: Contains the IP version number (4)• Header Length (IHL): Specifies the size of the packet header to know where

the header ends and hence data begins. • Packet Length: The entire packet size, including header and data. • Type-of-Service (TOS): reflect priority.• TTL: indicates the remaining "life" of the packet. TTL is decreased by at least

one each time the packet is processed by a router.• Identification: uniquely identifies fragments of an original IP packet.• Fragment offset: identifies the order in which to place the packet fragment in

the reconstruction. • MF & DF Flags• Header Checksum: used for error checking the packet header.• Protocol: Indicates the data payload type that the packet is carrying. Ex: 01

ICMP, 06 TCP, 17 UDP

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IP Fragmentation

• A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU.

• Packet does not get reconstructed until it reaches the host.If DF = 1, it will not fragment packet, but discards it.

• Fragment Offset field and MF flag are used to reconstruct the packet at the destination host.

Data = 1480 bytesIP

Data = 500IP

Data = 480IP

IP Packet Fragments

Original IP Packet

IP Header = 20 bytes

Data = 500IP

Data = 500L2 L2

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Dividing Networks

• large network should be separated into smaller networks that are interconnected.

• These smaller networks are often called subnetworks or subnets.

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Grouping Hosts Geographically

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Grouping Hosts for Purpose

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Grouping Hosts for Ownership

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

Layer 3 Devices

Router:- Best path determination- Creating routing table- Connecting different LANs

• All interfaces of the router are members in a multiple broadcast domains, and multiple collision domains.

• Supports more than one technology

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Device Parameters

• IP address• Subnet Mask• Default gateway• DNS Server

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

ARP

SIEMENSNIXDORF

SIEMENSNIXDORF

Host A

Host BIP Address: 128.0.10.4HW Address: 080020021545

ARP Reply

ARP Request - Broadcast to all hosts„What is the hardware address for IP address 128.0.10.4?“

SIEMENSNIXDORF

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

Thank You..