Mr. Mark Welton
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A computer network, or simply a network, is a
collection of computers and other hardware
interconnected by communication channels that
allow sharing of resources and information.
Where at least one process in one device is able
to send/receive data to/from at least one process
residing in a remote device, then the two devices
are said to be in a network. A network is a group
of devices connected to each other. Networks
may be classified into a wide variety of
characteristics, such as the medium used to
transport the data, communications protocol
used, scale, topology, benefit, and organizational
scope. - Wikipedia
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A computer network can be defined as two or more
computers connected by some means through which they are
capable of sharing information
So is sneaker net a network?
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A connection involves some sort of
addressing or identification of the nodes on
the network, allowing them to speak the
same protocol or language.
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Types of networks:
LAN - is a network that is confined to a limited
space, such as a building or floor.
WAN - A WAN is a network that is used to
connect LANs by way of a third-party provider.
CAN - is a network that connects LANs and/or
buildings in a discrete area owned
or controlled by a single entity.
MAN - is a network that connects LANs and/or
buildings in an area that is often
larger than a campus.
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We also describe networks by the technology they use
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Or the Protocols:
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Or the layer:
◦ Ethernet
◦ Token Ring
◦ FDDI (Fiber Distributed Data Interface)
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ATM
Frame Relay
MPLS
T1
DS3
◦ Bridged
◦ Routed
◦ Point-to-point
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Early Ethernet, 10Base-5 used a thick cable
named thick-net
10Base-2 Ethernet replaced 10Base-5 and
used a smaller coaxial cable named thin-net
Both used either an N or BNC connector to
couple cables together and required
terminators to be installed at the end of a
cable run
These cable runs formed the physical
backbone of Ethernet networks
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Ethernet running over UTP cables terminated
with RJ45 connectors made use of hubs
Hubs became the new backbone of many
installations
As hubs became less expensive, extra hubs
were often used a repeaters in more complex
networks
Hubs are layer one devices (physical/electrical
devices) and do not alter frames or make
decisions on traffic it sees
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A hub is simply a means of connecting
Ethernet cables together so that their signals
can be repeated to every other connected
cable on the hub
Hubs may also be called repeaters for this
reason, but it is important to understand that
while a hub is a repeater, a repeater is not
necessarily a hub.
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If another device is already using the wire, the
sending device will wait a bit and then try to
transmit again
When two stations transmit at the same time,
a collision occurs
Each station records the collision, backs off
again, and then retransmits
A collision domain is an area
of an Ethernet network where collisions can
occur
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A broadcast domain is the area of an Ethernet
network where a broadcast will be
propagated
Broadcasts stay within a Layer-3 network
(unless forwarded), which is usually bordered
by a Layer-3 device such as a router
A broadcast storm results in the attempt to
build redundancy into the network
The only way to resolve a broadcast storm is
to break the loop
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The 5-4-3 rule of Ethernet design states that
between any two nodes on an Ethernet
network, there can be only five segments,
connected via four repeaters, and only three
of the segments can be populated. This rule,
which seems odd in the context of today’s
networks, was the source of much pain for
those who didn’t understand it.
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Switches differ from hubs by playing an active
role in the processing of frames on the
network
Switch - The general term used for anything
that can switch
Ethernet Switch - Any device that forwards
frames based on their Layer-2 MAC
addresses using Ethernet
A switch creates a collision domain on each
port, while a hub’s collision domain exists on
all ports
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Layer-3 Switch - This is a switch with routing
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Multilayer Switch - Similar to a Layer-3
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capabilities
switch, but may also allow for control based
on higher layers in packets. Multilayer
switches allow for control based on TCP, UDP,
and even details contained within the data
payload of a packet
Switching - is the act of forwarding frames
based on their destination MAC addresses
 Every frame contains the source and destination MAC address
 switch inspects the frame to determine the source MAC address and
adds the address into its address table
 The switch then determines the frame’s destination MAC address and
checks the table
 If a match exists, the frame is forwarded from the source port to the
destination port. If no match is found, the frame is sent to all ports
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Routing - refers to determining the path for
something
For IP, packets and frames are forwarded by
switches and hubs
To reach a destination, not on the local
network, the packet must be forwarded to a
gateway
The gateway (router) is responsible for
determining how to get the packet to where it
needs to go
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Routers communicate with each other by
routing protocols
These protocols let the routers learn
information about other networks which are
stored in the routing table of the router
With the introduction of layer three switches –
routers tend to remain WAN-focused, while
switches remain LAN-focused
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Routing is a fundamental process common to
almost every network
Each routing protocol has its own table of
information (learned through its own protocol –
in addition to the routing table of the router)
Metrics - the values or weight used by a routing
protocol to determine its best path to a
destination
Administrative distance is the values assigned to
each routing protocol by the router in order to
prioritize learned routes
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If the same route is learned from two sources
within a single routing protocol, the one with
the best metric will win
If the same route be learned from two routing
protocols within a single router, the protocol
with the lowest administrative distance will be
preferred
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When a packet arrives at a router, the router
determines whether the packet needs to be
forwarded to another network
If it does, the router checks the routing table to
determine if it contains a route to the destination
network
If there is a match, the packet is forwarded out
the proper interface to where it belongs
If there is no match found in the routing table,
the packet is forwarded to the default gateway, if
one exists, or dropped if no default gateway
exists
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IP destination networks are often described
by a network address and prefix length
The prefix length is the number of bits set to
1 in the subnet mask
Networks are described in the format
network-address/prefix-length
i.e. 10.0.0.0/16
referred to as a prefix
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10.0.0.0/24
10.0.0.0/16
10.0.0.0/8
Which has the longer prefix?
A longer prefix means a more specific match
to a destination
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Host route
subnet route
summary route
major network route
supernet route
default route
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Host route is a route to a host (node), not a
network
Host routes have a subnet mask of
255.255.255.255 or a /32 prefix
The subnet mask is all 1s
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Subnet route is a portion of a major network
The subnet mask is used to determine the
network
24 is a subnet of the classful 10.0.0.0
network
Subnets make use of Variable Length Subnet
Masks of classful major networks
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Summary route is a single route that
references a group of subnets
10.1.0.0/16 would be a summary route for
the subnet routes of 10.1.1.0/24 and
10.1.2.0/24
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is any classful network with its traditional
subnet mask
Class A, B, C networks
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Supernet route is a single route that
references a group of major network
192.168.0.0.0/15 is a supernet for
192.168.0.0.0/16 and 192.169.0.0/16
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Default route is the gateway (route) of last
resort
typically shown as 0.0.0.0 (an all zero)
destination
The subnet mask is all zeroes