Module 10
Routing Fundamentals
and Subnetting
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Routable Protocols
• A protocol is a set of rules that determines how computers
communicate with each other across networks
• A protocol describes the following:
– The format that a message must conform to
– The way in which computers must exchange a
message within the context of a particular activity
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Routed Protocols
• A routed protocol allows the router to forward data
between nodes on different networks.
– it must have the ability to assign a network number and
a host number to each individual device
• These protocols also require a network mask (subnet
mask) in order to differentiate between the network and
host portion of the address.
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Routable Protocols
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Encapsulation Process
Data
Segments
Packets
Frames
Bits
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Transport Layer
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Internet Protocol (IP)
• The Internet Protocol (IP) is the most widely used
network-addressing scheme
• IP is a connectionless, unreliable, best-effort delivery
protocol
• IP does not verify that the data reaches its
destination; this function is handled by the upper
layer protocols
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IP
• Data is encapsulated into packets, also known as
datagrams at the network layer of the OSI model.
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Connection-Oriented vs. Connectionless
Networks
• Connectionless Networks
– Often referred to as packet-switched processes
– IP Protocol
– Internet
• Connection-Oriented Networks
– Often referred to as circuit-switched processes
– A connection with the recipient is first established, and
then data transfer begins
– TCP adds Layer 4, connection-oriented reliability
services to IP
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Network Layer
• Routing is an OSI Layer 3
function
• Routing is the process of
finding the most efficient
path from one device to
another
• The primary device that
performs the routing process
is the router
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Routers
• A router is a network
layer device that uses
one or more routing
metrics to determine the
optimal path along which
network traffic should be
forwarded
• Routers make logical
decisions regarding the
best path for the delivery
of data
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Routing metrics are values
used in determining the
advantage of one route over
another
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Routers
• The encapsulation and de-encapsulation process occurs each time a
packet transfers through a router
• This process breaks up the data stream into segments, adds the
appropriate headers and trailers then transmits the data. The deencapsulation process is the opposite process, removing the headers
and trailers, then recombining the data into a seamless stream
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Routing versus Switching
• Switching occurs at Layer 2 of
the OSI model
• Routing occurs at Layer 3
• The Layer 2 switch can only
recognize its own local MAC
addresses and cannot handle
Layer 3 IP addresses
• A Layer 2 switch interconnects
segments belonging to same
logical network or subnetwork
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Routing versus Switching
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Routing versus Switching
• Each computer and router interface maintains an ARP table for Layer
2 communication. The ARP table is only effective for the broadcast
domain (or LAN) that it is connected to.
• The router also maintains a routing table that allows it to route data
outside of the broadcast domain
• Each ARP table contains an IP-MAC address pair
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Routing versus Switching
• The Layer 2 switch can only recognize its own local MAC
addresses and cannot handle Layer 3 IP addresses
• When a host has data for a non-local IP address, it sends the
frame to the closest router also known as its default gateway
• The host uses the MAC address of the router as the destination
MAC address
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Routing versus Switching
• Another difference between switched and routed networks
is switched networks do not block broadcasts
• Routers block LAN broadcasts
• Because routers block broadcasts, routers also provide a
higher level of security and bandwidth control than
switches
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Subnetting
• Subnetting provides manageability, enables the network
administrator to provide broadcast containment, and low-level
security on the LAN.
• Subnet addresses include the Class A, Class B, and Class C
network portion, plus a subnet field and a host field. The subnet
field and the host field are created from the original host portion
of the major IP address.
• A LAN is seen as a single network with no knowledge of the
internal network structure. This view of the network keeps the
routing tables small and efficient.
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Subnet Mask
• The subnet mask gives the router the information required to
determine in which network and subnet a particular host resides
• The subnet octet or octets are determined by adding the
position value of the bits that were borrowed. If three bits were
borrowed, the mask for a Class C address would be
255.255.255.224.
• This mask may also be represented, in the slash format, as /27.
The number following the slash is the total number of bits that
were used for the network and subnetwork portion.
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Subnetting Formula
• Number of usable subnets equals two to the power of the
assigned subnet bits or borrowed bits, minus two (reserved
addresses for subnetwork id and subnetwork broadcast)
2 power of borrowed bits –2 = usable subnets
23 = 8 - 2 = 6 usable subnets
• Number of usable hosts equals two to the power of the bits
remaining, minus two (reserved addresses for subnet id and
subnet broadcast)
2 power of remaining host bits –2 = usable hosts
25 = 32 – 2 = 30 usable hosts per subnet
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ANDing Process
• Routers use subnet masks to determine the home subnetwork
for individual nodes. This process is referred to as logical
ANDing.
• ANDing is a binary process by which the router calculates the
subnetwork ID for an incoming packet.
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