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Network Layer
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In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers.
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NETWORK LAYER
ADDRESSINGthe Layer
ELEMENTS
Local Network Address
category
publicly concerned (TCP/IP)
private
IPX/SPX, SNA…
unconnected TCP/IP networks
Service Address
inside Local Network Address
identify numbers oftargeted machines
socket
port numbers
many purposes
http: 80
https: 8080
ftp: 21
sftp: 22
private printing: 35
telnet: 23
smtp: 25
DNS: 53
...
Network address + Service Address ==> uniquely identify the connection
SWITCHING
def
the way packets are routed
within connected network
type
CIRCUIT switching
maintain connection during the session
MESSAGE switching
one hop at a time
PACKET switching
data packets sent thru multiple routes
at destination
assembled
checked
collected
read
ROUTERS
purpose
the mechanism used to determine THE PATH
connect 2 networks
via a series of intermediate networks
maintain connectivity among adjacent networks
ROUTE TABLE
def
determine the path, used to connect 2 networks
store
in
router
networked computer
what?
the topology of the network
the routes to particular network destinations
MAINTAIN "table" identifies networks it attaches to availability at an interface
router share with others abt networks it knows
they build up a 'route' to connect to a certain networks
HOPS
distances to connect routers
ROUTEDISCOVERY
Number of hops = distance vector
discover networks by exchanging router tables
ROUTESELECTION
dynamic
when a decision about the route is made at each connection
static
the preferred route for all communications between 2 networks
If one router goes down then all interconnected networks become unavailable
GATEWAY SERVICES
Network Layer translation
relay packets thru different networks
CONNECTIONSERVICE
Network-Layer flow control
Error control
Packet sequence control
