This document provides an overview of the transport and network layers of the OSI model, detailing functions like end-to-end data transfer, addressing, and packet handling using protocols such as IPv4 and IPv6. It discusses key concepts like IP packet characteristics, fragmentation, and the routing of data through various network devices, particularly routers. Additionally, it emphasizes the importance of structuring large networks into smaller subnetworks for efficient communication.

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Network Fundamentals
Abdelkhalik Elsaid Mosa
abdu.elsaid@yahoo.com
abdelkhalik.staff.scuegypt.edu.eg

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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.Addressing
2.Encapsulation
3.Routing
4.Decapsulation

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Network Layer Protocols
• Routed Protocols: carry user data
1. 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 destination
1. RIP, BGP “application layer Protocols”
2. IGRP, EIGRP
3. OSPF
4. IS-IS
• Other supporting protocols
1. ICMP
2. 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.

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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 size
of a packet.
• The Data Link layer passes
the 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

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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

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ARP
SIEMENS
NIXDORF
SIEMENS
NIXDORF
Host A
Host B
IP Address: 128.0.10.4
HW Address: 080020021545
ARP Reply
ARP Request - Broadcast to all hosts
„What is the hardware address for IP address 128.0.10.4?“
SIEMENS
NIXDORF

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