OSI Network Layer
Network Fundamentals – Chapter 5
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Objectives

Identify the role of the Network Layer, as it describes
communication from one end device to another end device

Examine the most common Network Layer protocol,
Internet Protocol (IP), and its features for providing
connectionless and best-effort service

Understand the principles used to guide the division or
grouping of devices into networks

Understand the hierarchical addressing of devices and
how this allows communication between networks

Understand the fundamentals of routes, next hop
addresses and packet forwarding to a destination network
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Course Index
 5.1 IPv4
 5.2 Networks – Dividing Hosts into Groups
 5.3 Routing – How Our Data Packets are Handled
 5.4 Routing Processes: How Routes are Learned
 5.5 Lab Activities
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5.1 IPv4
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5.1.1 Network Layer – Communication from Host to Host
 Network Layer
Provides services to exchange the individual pieces of data over the
network between identified end devices
Basic process:
Addressing
Encapsulation
Routing
Decapsulation
Protocols
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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5.1.2 The IPv4 Protocol
 Role of IPv4
It is used to carry user data over the Internet
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5.1.3 The IPv4 Protocol – Connectionless
 Connectionless Service
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5.1.4 The IPv4 Protocol – Best Effort
 Best Effort Service (unreliable)
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5.1.5 The IPv4 Protocol – Media Independent
 Media independent


Maximum Transmission Unit (MTU)：the maximum size of PDU that each medium can
transport.
fragmenting the packet or fragmentation： the process of an intermediary device usually a router - will need to split up a packet when forwarding it from one media to a
media with a smaller MTU
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5.1.6 IPv4 Packets – Packaging the Transport Layer PDU
 IPv4 encapsulates, or packages, the Transport layer
segment or datagram so that the network can deliver it
to the destination host.
 The encapsulated Transport layer PDU - remains
unchanged during the Network layer processes.
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5.1.7 IPv4 packet header
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5.1.7 IPv4 packet header
 Typical IP Packet
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5.2 Networks – Dividing Hosts into Groups
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5.2.1 Networks –
Separating Hosts into Common Groups
 Subnet
Separating hosts into common groups
The factors of group
Geographic location
Purpose
Ownership
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5.2.2 Why Separate Hosts Into
Networks?
 Performance
Dividing large networks so that hosts who need to communicate
are grouped together reduces the traffic across the
internetworks.
Broadcasts are contained within a network, so a network is also
known as a broadcast domain.
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5.2.3 Why Separate Hosts Into Networks?
 Security
Dividing networks based on ownership means that access to
and from resources outside each network can be prohibited,
allowed, or monitored.
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5.2.4 Why Separate Hosts Into
Networks?
 Address Management
Dividing large networks so that hosts who need to communicate
are grouped together reduces the unnecessary overhead of all
hosts needing to know all addresses
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5.2.5 How to divide networks?
 Hierarchical addressing
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5.2.6 Dividing the Networks – Networks from Networks
 IPv4 is a Hierarchical addressing
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5.3 Routing – How Our Data Packets are
Handled
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5.3.1 Device Parameters
- Supporting Communication Outside Our Network
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5.3.2 IP packet—carrying data end to end
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5.3.3 A Gateway – The Way Out of Our Network
 A router interface connected to the local network.
 When a packet’s destination is a different network, the
packet is sent to the gateway.
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5.3.4 A Route – The Path to Network
 Three main features of route:
Destination network
Next-hop
Metric
 Routing table
 Routing protocols.
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5.3.5 Destination network
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5.3.6 The Next Hop – Where the Packet
Goes Next
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5.3.7 Packet Forwarding
 Forward it to the next-hop router
Have a exactly entry
No exactly entry but a default route exits
 Forward it to the destination host
Direct connected network
 Drop it
No entry and no default route
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5.4 Routing Processes: How Routes are
Learned
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5.4.1 Routing Protocols
 Used for Sharing the Routes
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5.4.2 Static Routing
 Manually configured on the router
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5.4.3 Dynamic Routing
 Dynamic Routing protocols are the set of rules by which routers
dynamically share their routing information.
 Common dynamic routing protocols:
Routing Information Protocol (RIP)
Enhanced Interior Gateway Protocol (EIGRP)
Open Shortest Path First (OSPF)
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Summary
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