Class notes
Cisco Modules 8-10 Notes
- Course
- ITCC 1414
- Institution
- San Antonio College
The document contains notes related to modules 8-10 on Netacad.
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8 Network Layer CharacteristicsThe Network Layer (OSI Layer 3) provides services that allow end devices to
exchange data across networks
IPv4 and IPv6 are the principal network layer communication protocols
Other protocols include routing protocols like Open Shortest Path First
(OSPF) and messaging protocols like Internet Control Message Protocol
(ICMP)
4 Basic Operations Network Layer Protocols must perform:
Addressing End Devices
o End devices must be configured with a unique IP address for
identification on the network
Encapsulation
o Network layer encapsulates the protocol data unit (PDU) from the
transport layer into a packet
o Adds IP header information (Ex. The IP address of the source and
destination hosts)
o Encapsulation process is performed by the source of the packet
Routing
o Network layer provides services to direct the packets to a
destination host on another network
o To travel to another network, a packet must pass through a
router
o The router selects the best path and directs packets towards
the destination host
, Process is known as Routing
o A packet may cross through many routers before reaching the
destination host
De-Encapsulation
o When the packet arrives at the destination host, the host checks
the IP address & if the IP address matches the address the hosts
IP address then the header is removed
o After the packet is de-encapsulated the Layer 4 PDU is passed to
the appropriate service at the transport layer
De-encapsulation is performed by the destination host of
the packet
Hop – Each router a packet crosses to reach the destination host
Network layer communication protocols specify the packet structure and
processing used to carry the data from 1 host to another
Operates without regard to the data carried in each packet
o Allows the network layer to carry packets for multiple
communication types between multiple hosts
IP encapsulated the transport layer segment/other data by adding an IP
header
Used to deliver the packet to the destination host
Figure shows how the transport layer PDU is encapsulated by the network
layer PDY to create an IP packet
Encapsulating data layer by layer allows the services at the different layers
to develop and scale without affecting the other layers
Means that the transport layer segments can be packaged by IPv4 or
IPv6 by any new protocol
,IP header is examined by Layer 3 devices (ex. Routers and switches) as it
travels
NOTE: The IP addressing information remains the same from the time the
packet leaves the source until it arrives at its destination, except for when
translated by the device performing Network Address Translation (NAT) for
IPv4
Routers implement routing protocols to route packets between networks
The routing performed by these devices examines the network layer
addressing in the packet header
In all cases, the data portion of the packet remains unchanged during the
network layer processes
IP only provides the functions that are necessary to deliver a packet from a
source to a destination over a system of networks
IP was not designed to track and manage the flow of packets
o If needed, other functions are performed by other layers (usually
TCP at layer 4)
Basic IP Characteristics:
Connectionless
o No connection with the destination established before sending
data packets
Best Effort
o IP is inherently unreliable because packet delivery is not
guaranteed
Media Independent
o Operation is independent of the medium (ex. Copper, Fiber-Optic,
or Wireless) carrying the data
Connectionless – No dedicated end-to-end connection is created by IP before
data is sent
Connectionless communication is like sending a letter to someone
without notifying the recipient
Requires no initial exchange of control information to establish an end-
to-end connection before the packets are forwarded
IP does not require additional fields in the header to maintain an established
connection
, Greatly reduces the overhead of IP (good thing)
No pre-established end-to-end connection means that senders are unaware if
the destination devices are present/functional when the packet is sent
They are also unaware if the destination receives the packet
OR if the destination device can access and read the packet
IP protocol does not guarantee that all packets that are packets that get
delivered are received
Figure shows the unreliable/best-effort delivery characteristics of the IP
protocol
IP does not have the capability to manage and recover from
undelivered/corrupt packets
IP packets are sent with information about the location of the delivery
but DO NOT contain information that can be processed to inform the
sender if the delivery was successful
Packets may arrive corrupted, out of order, or not at all and IP has no
ability for packet re-transmissions if these errors happen
If packets are delivered out of order (or if some are missing) the upper layer
services must resolve the issue
Reliability is the role of the TCP protocol at the transport layer
IP packets can be communicated as electronic signals over a copper cable,
as optical signals over fiber, or wirelessly as radio signals
Operates independently of the media that carries the data at lower
layers of the protocol stack
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