CCNA – Semester1
Chapter 5 - OSI Network Layer
CCNA Exploration version 4.0
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
Introduction
• The protocols of the OSI model Network layer specify
addressing and processes that enable Transport layer data to
be packaged and transported. The Network layer encapsulation
allows its contents to be passed to the destination within a
network or on another network with minimum overhead.
IPv4
Network Layer – Communication from Host to
Host
• Layer 3 uses four basic processes:
– Addressing
– Encapsulation
– Routing: Intermediary devices that connect the networks
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.
– Decapsulation
Network Layer – Communication from Host
to Host
Network Layer – Communication from Host
to Host
Network Layer Protocols
• 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)
The IPv4 Protocol – Example Network Layer
Protocol
• The Internet Protocol 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.
The protocol was not designed to track and manage the flow of
packets. These functions are performed by other protocols in other
layers. Basic characteristics:
The IPv4 Protocol – Connectionless
The IPv4 Protocol – Best Effort
• Since protocols at other layers can manage reliability, IP is
allowed to function very efficiently at the Network layer.
Best Effort Service (unreliable)
• Describe the implications for the use of the IP protocol as it is considered an unreliable protocol
• Unreliable means simply that IP does not have the capability to manage, and recover from, undelivered or corrupt packets.
The IPv4 Protocol – Media Independent
• One major characteristic of the media that the Network layer considers: the maximum size of PDU that each medium can transport: the Maximum Transmission Unit (MTU). Part of the control communication between the Data Link layer and the Network layer is the establishment of a maximum size for the packet.
• IPv4 and IPv6 operate independently of the media that carry the data at lower layers of the protocol stack
Packaging the Transport Layer PDU
• The process of encapsulating data by layer enables the services at the
different layers to develop and scale without affecting other layers.
• Routers can implement these different Network layer protocols to operate
concurrently over a network to and from the same or different hosts. The
routing performed by these intermediary devices only considers the
contents of the packet header that encapsulates the segment.
IPv4 Packet Header
Network Layer Fields
• 4 bits
• Indicates version of IP used
• IPv4: 0100; IPv6: 0110
Network Layer Fields
• 4 bits
• Indicates datagram header length in 32 bit words
Network Layer Fields
• 8 bits
• Specifies the level of importance that has been
assigned by upper-layer protocol
Network Layer Fields
• 16 bits
• Specifies the length of the entire packet in bytes,
including data and header
Network Layer Fields
• 16 bits
• Identifies the current datagram
Network Layer Fields
• 3 bits
• The second bit specifies if the packet can be fragmented; the last
bit specifying whether the packet is the last fragment in a series
of fragmented packets.
Network Layer Fields
• 13 bits
• Used to help piece together datagram
fragments
Network Layer Fields
• 8 bits
• Specifies the number of hops a packet may travel. This
number is decreased by one as the packet travels
through a router
Network Layer Fields
• 8 bits
• Indicates which upper-layer protocol, such as TCP(6) or
UDP(17), receives incoming packets after IP
processing has been completed
Network Layer Fields
• 16 bits
• Helps ensure IP header integrity
• Not caculated for the encapsulation data
Network Layer Fields
• 32 bits
• Specifies the sending node IP address
Network Layer Fields
• 32 bits
• Specifies the receiving node IP address
Network Layer Fields
• Variable length
• Allows IP to support various options, such as security
Network Layer Fields
• Variable length
• Extra zeros are added to this field to ensure that the IP
header is always a multiple of 32 bits.
Network Layer Fields
• Variable length up to 64 KB
• Contains upper-layer information
Networks –
Dividing Hosts into Groups
Separating Hosts into Common Groups
• Networks can be grouped based on factors that include:
– Geographic location
– Purpose
– OwnershipGeographic
Separating Hosts into Common Groups
Purpose: Users who have similar tasks typically use
common software, common tools, and have common
traffic patterns.
Separating Hosts into Common Groups
Purpose
Separating Hosts into Common Groups
Ownership: Using an organizational (company,
department) basis for creating networks assists in
controlling access to the devices and data as well as
the administration of the networks.
Separating Hosts into Common Groups
Ownership
Why separate hosts into networks
Common issues with large networks are: Performance
degradation, Security issues, Address Management
• Improving Performance:
Why separate hosts into networks
• Increase network security
Why separate hosts into networks
• Address management: To expect each host to know the
address of every other host would impose a processing
burden on these network devices that would severely
degrade their performance.
Why separate hosts into networks
• Hierarchical addressing: solves the problem of devices
communicating across networks of networks
Dividing the networks - Networks from networks
• If a large network has to be divided, additional layers of
addressing can be created. Using hierarchical addressing
means that the higher levels of the address are retained; with a
subnetwork level and then the host level.
Routing –
How Our Data Packets are Handled
Routing Protocols
• Routing is an OSI Layer 3
function. It is a hierarchical
scheme and allows individual
addresses to be group together.
• Routing is the process of finding
the most efficient path from one
device to another.
Routing Protocols
• Provides processes for sharing route information
• Allows routers to communicate with other routers to update and maintain
the routing tables
• Examples: Routing Information Protocol (RIP), Interior Gateway Routing
Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway
Protocol (BGP), and Enhanced IGRP (EIGRP)
Supporting communication outside our network
This is called the next-hop address. If a route is available to the router, the router will forward the packet to the next-hop router that offers a path to the destination network.
• To communicate with a device on another network, a host
uses the address of this gateway, or default gateway, to
forward a packet outside the local network.
• The router also needs a route that defines where to forward
the packet next.
Fundamentals of Routes, Next Hop Addresses and Packet
Forwarding
• If the destination host is in the same network as the source host, the packet is delivered between the two hosts on the local media without the need for a router.
• If the destination host and source host are not in the same network, the packet may be carrying a Transport layer PDU across many networks and through many routers.
IP Packet – Carrying Data End-to-End
IP Packet – Carrying Data End-to-End
IP Packet – Carrying Data End-to-End
IP Packet – Carrying Data End-to-End
IP Packet – Carrying Data End-to-End
A gateway – the way out of our network
A gateway – the way out of our network
A gateway – the way out of our network
• A router makes a forwarding decision for each packet that arrives at the
gateway interface. This forwarding process is referred to as routing. To
forward a packet to a destination network, the router requires a route to
that network. If a route to a destination network does not exist, the packet
cannot be forwarded.
Routing table
• The routing table stores information about connected and
remote networks. Routes in a routing table have three main
features:
– Destination network
– Next-hop
– Metric
A Route – The Path to a Network
Host Routing Table
• A host creates the routes used to forward the packets it originates. These routes are derived from the connected network and the configuration of the default gateway.
• Hosts automatically add all connected networks to the routes. These routes for the local networks allow packets to be delivered to hosts that are connected to these networks.
Route print
Routing table entries
Routing table entries
Default route
• A router can be configured to have a default route. A 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. The
default route is used to forward packets for which there is no entry
in the routing table for the destination network. Packets with a
destination network address that does not match a more specific
route in the routing table are forwarded to the next-hop router
associated with the default route.
Packet forwarding
• Routing is done packet-by-packet and hop-by-hop. Each
packet is treated independently in each router along the path.
• The router will do one of three things with the packet: Forward
it to the next-hop router; Forward it to the destination host;
Drop it.
Packet forwarding
• If the routing table does not contain a more specific route entry for an
arriving packet, the packet is forwarded to the interface indicated by a
default route, if one exists. The default route is also known as the
Gateway of Last Resort.
Packet forwarding
Routing Processes –
How Routes are Learned
Routing protocol – Sharing the route
• Routing protocols: static and dynamic routes
Static Routing
• Static route: routes to remote networks with the
associated next hops can be manually configured on the
router. A default route can also be statically configured.
Dynamic Routing
• Routing protocols are the set of rules by which routers
dynamically share their routing information.
Routing protocol
• Provides processes for sharing route information
• Allows routers to communicate with other routers to update and
maintain the routing tables
• Examples: Routing Information Protocol (RIP), Interior Gateway
Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border
Gateway Protocol (BGP), and Enhanced IGRP (EIGRP)
IGP and EGP
• Autonomous system is a network or set of networks under common
administrative control. An autonomous system consists of routers that present
a consistent view of routing to the external world.
• Interior Gateway Protocols (IGP): route data within an autonomous system. Eg:
RIP and RIPv2; IGRP; EIGRP; OSPF; IS-IS;
• Exterior Gateway Protocols (EGP): route data between autonomous systems.
Eg: BGP
Link state and Distance Vector
• The distance-vector routing approach determines the
distance and direction, vector, to any link in the
internetwork. Routers using distance-vector algorithms send
all or part of their routing table entries to adjacent routers on
a periodic basis. This happens even if there are no changes
in the network. Eg: RIP, IGRP, EIGRP
• Link state routing protocols send periodic update at longer
time interval (30’), Flood update only when there is a
change in topology. Link state use their database to create
routing table. Eg: OSPF, IS-IS
Dynamic Routing: Example 5.4.3.2
Summary
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