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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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IP Header
• IP header length (HLEN) – Indicates the datagram header length in
32-bit words.
• Padding – extra zeros are added to this field to ensure that the IP
header is always a multiple of 32 bits.
• Flags – A three-bit field in which the two low-order bits control
fragmentation
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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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Routed versus Routing Protocols
• Routed protocols transport data
across a network
• Examples of routed protocols
include:
– Internet Protocol (IP)
– Novell's Internetwork
Packet Exchange (IPX)
– DECnet
– AppleTalk
– Xerox Network Systems
(XNS)
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Routed versus Routing Protocols
• Routing protocols allow routers to
choose the best path for data from
source to destination
• Routers use routing protocols to
exchange routing tables and share
routing information
• Routing protocols enable routers to
route routed protocols
• Examples of routing protocols
include:
– Routing Information Protocol (RIP),
Interior Gateway Routing Protocol
(IGRP), Open Shortest Path First
(OSPF), Border Gateway Protocol
(BGP), and Enhanced IGRP
(EIGRP).
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Path Determination
• Path determination occurs at the network layer
• It enables a router to compare the destination address to the
available routes in its routing table, and to select the best path
• The routers learn of these available routes through static
routing or dynamic routing
– Routes configured manually by the network administrator are
static routes
– Routes learned by others routers using a routing protocol are
dynamic routes
• Each router that the packet encounters along the way is called
a hop. The hop count is the distanced traveled
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Routing Tables
• Routers use routing protocols to build and maintain routing
tables that contain route information
• Routers keep track of important information in their routing
tables, including the following:
– Protocol type – The type of routing protocol that created the
routing table entry
– Destination/next-hop associations
– Routing metric
– Outbound interfaces
• Routers communicate with one another to maintain their routing
tables through the transmission of routing update messages
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Routing algorithm and metrics
• Routing protocols often have one or more of the following
design goals:
– Optimization
– Simplicity and low overhead
– Robustness and stability
– Flexibility
• Routing algorithms use different metrics to determine the best
route. The most commonly used metrics include:
– Bandwidth Delay
– Load Reliability
– Hop count
– Ticks
– Cost
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IGPs and EGPs
• Two types of routing protocols are Interior Gateway Protocols
(IGPs) and Exterior Gateway Protocols (EGPs)
• IGPs route data within an autonomous system.
• EGPs route data between autonomous systems
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IGPs and EGPs
• Examples of IGPs include:
– Routing Information Protocol (RIP) and (RIPv2)
– Interior Gateway Routing Protocol (IGRP)
– Enhanced Interior Gateway Routing Protocol (EIGRP)
– Open Shortest Path First (OSPF)
– Intermediate System-to-Intermediate System Protocol (ISIS)
• An example of EGP is:
– Border Gateway Protocol (BGP)
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Link State and Distance Vector
• IGPs can be further categorized as either distance-vector or
link-state protocols
• Distance-vector routing determines the distance and direction
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
• Link-state algorithms typically use their databases to create
routing table entries that prefer the shortest path
– Link-state routing protocols respond quickly to network
changes sending trigger updates only when a network
change has occurred
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Distance-vector Routing Protocols
• Examples of distance-vector protocols include:
– Routing Information Protocol (RIP) – The most common IGP
in the Internet, RIP uses hop count as its only routing metric.
RIP cannot route a packet beyond 15 hops.
– Interior Gateway Routing Protocol (IGRP) – This IGP was
developed by Cisco to address issues associated with routing
in large, heterogeneous networks. IGRP can select the fastest
available path based on delay, bandwidth, load, and reliability.
IGRP also has a much higher maximum hop count limit than
RIP.
– Enhanced IGRP (EIGRP) – This Cisco-proprietary IGP
includes many of the features of a link-state routing protocol.
EIGRP provides superior operating efficiency such as fast
convergence and low overhead bandwidth Because of this, it
has been called a balanced-hybrid protocol, but it is really an
advanced distance-vector routing protocol
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Link-State Routing Protocols
• Examples of link-state protocols include:
– Open Shortest Path First (OSPF)
– Intermediate System-to-Intermediate System (IS-IS). used
for routed protocols other than IP
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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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