This document provides an overview of routing fundamentals and subnets. It defines key concepts like routed protocols, routing protocols, IP addressing, and subnetting. Routed protocols like IP define packet formats and addressing to enable communication across networks, while routing protocols like RIP exchange information to maintain routing tables and select optimal paths. The document also explains how routers use routing tables to determine the best path for sending packets and contains examples of subnetting IP addresses to create multiple subnets within a class C network.

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Module 10
Routing Fundamentals and
Subnets

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Objectives

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IP Address

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IP Address Grouping

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Routed and Routing Protocols
• Consider that a packet needs to be sent from node A
to node F. How would it decide which path to take?

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Routing Protocol vs Routed Protocol
• A routed protocol
1. defines the end to end addressing and the
packet format of a packet that is forwarded
between nodes on different networks.
Internet Protocol (IP) is a routed protocol
• A routing protocol
1. exchanges topology information with adjacent
routers to update and maintain their routing tables.
2. selects the best path through a network
RIP is a routing protocol

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Routed Protocol
• A protocol is a set of rules
• A routed protocol is a set of rules that determines how
computers at the source and destination communicate
with each other across networks
– packet format
– end to end addressing
• In order for a protocol to be routable, it must provide the
ability to assign both a network number and a host
number for each individual device.

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Internet Protocol IP
• IP is a connectionless, unreliable, best-effort delivery protocol
• As information flows down the layers of the OSI model, the data is
processed at each layer.
• IP accepts whatever data is passed down to it from the upper layers.

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IP Packet Header

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Network Layer Devices in Data Flow
• As a frame is received at a
router interface.
• The MAC address is checked
to see if the frame is directly
addressed to the router
interface, or a broadcast.
• The frame header and trailer
are removed and the packet is
passed up to Layer 3.
• The destination IP address is
compared to the routing table
to find a match.
• The packet (datagram) is
placed in a new frame with the
MAC address of the next hop
interface.
• The frame is then transmitted.
If a match is found or there is a default
route, the packet will be sent to the
interface specified in the matched routing
table statement otherwise packet is
discarded

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Packets Travel Across Links in a Frame
• Packets NEVER travel through the network – they are carried within frames
• A new frame MUST be created to carry the packet over each individual link
• Routers provide the IP address of the next hop interface (router or host)
• The ARP table provides the MAC address of this IP address for the frame destination

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Connectionless vs. Connection-Oriented
• In a connection oriented system is established
between the sender and the recipient before any
data is transferred.
– example: Telephone
• In a connectionless system, the destination is not
contacted before a packet is sent.
– example: Postal system
• TCP is connection oriented
• IP is connectionless

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Connectionless Network Services
• The Internet is a huge network where packets are routed according to their
IP addresses.
• IP is unreliable and best-effort as IP does not verify that the data reached its
destination and therefore does not resend missing packets.
• Reliability and resending of packets is handled by the upper layer protocols.
• IP may be used in conjunction with TCP to add a Layer 4, connection-
oriented service that checks for missing segments and resends them to
provide reliability.

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The IPv4 Packet Header
Time-to-live (TTL)
Count Decreases with every hop
This prevents packets from
looping endlessly.

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Routing

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The Network Layer

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Routing
• Routing is an OSI Layer 3 function.
• Routers connect networks (or subnetworks)
• Routing is the process of finding the most efficient
path from one device to another (router)
• Routers must maintain routing tables and make sure
other routers know of changes in the network
topology. This function is performed using a routing
protocol to communicate network information with
other routers

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Routing Through a Network
• A router is a network layer device that uses one or more
routing metrics to determine the optimal path through the
network

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Routing Metrics

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Data Encapsulation

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Layer 3 Routing and Layer 2 Switching

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Routers Reduce the Size of Broadcast Domains
• Routers block LAN broadcasts, so a broadcast
storm only affects the broadcast domain from which
it originated
• Switched networks do not block broadcasts

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Routing and Switching in a Network

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ARP Tables and Routing Tables

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The difference between a routed and routing
protocol – revisited

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Routed Protocol

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Routing Protocol

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Routed Vs Routing protocols
• A Routed Protocol:
– A network protocol suite that provides enough information in its network
layer address to allow a router to forward it to the next device and ultimately
to its destination.
– Defines the format and use of the fields within a packet.
– The Internet Protocol (IP) and Novell's Internetwork Packet Exchange (IPX),
DECnet, AppleTalk, Banyan VINES, and Xerox Network Systems (XNS)
• A Routing Protocol:
– Provides processes for sharing route information. Exchange topology info.
To determining the best routing paths and transporting packets through an
internetwork
– Also allows routers to communicate with other routers to update and
maintain the routing tables.
– 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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Back to Routing

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Path Determination

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Path Determination

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Routing Tables
• Routing tables contain the best routes to all known
networks.
• These routes can be either
– Static routes, which are entered manually by the system
administrator
– Or dynamic routes, which are constructed from
information passed between adjacent routers.
• A routing table entry contains:
– Each Destination
– The next hop IP address to reach that destination
– The metric for the route via that next hop
– Outbound router interface for the next hop

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Routing Tables

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Routing Algorithms and Metrics
• Routing protocols have one or more of the following
design goals:
Optimization
Simplicity and low overhead
Robustness and stability
Flexibility
Rapid convergence

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Routing Algorithms and Metrics

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Interior and Exterior Gateway Protocols

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Interior and Exterior Gateway Protocols
• IGPs route data within an autonomous system.
RIP, RIPv2, IGRP, EIGRP, OSPF, IS-IS
• EGPs route data between autonomous systems
Border Gateway Protocol (BGP)

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Interior Gateway Routing Protocols
• Link State and Distance Vector Routing Protocols
• Examples of distance-vector protocols:
Routing Information Protocol (RIP)
Interior Gateway Routing Protocol (IGRP)
Enhanced IGRP (EIGRP)
• Examples of link-state protocols:
Open Shortest Path First (OSPF)
Intermediate System-to-Intermediate System (IS-IS)

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Mechanics of Subnetting

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Subnetting
• Reasons for subnetting
– Provides addressing flexibility for the network
administrator.
• Each LAN must have its own network or subnetwork
address.
– Provides broadcast containment and low-level security on
the LAN.
– Provides some security since access to other subnets is
only available through the services of a router.

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IP Address Bit Patterns

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Introduction to Subnetting
• Host bits must are
reassigned (or
“borrowed”) as
network bits.
• The starting point is
always the leftmost
host bit.
3 bits borrowed allows 23
-2 or 6 subnets
5 bits borrowed allows 25
-2 or 30 subnets
12 bits borrowed allows 212
-2 or 4094 subnets

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Subnetting Chart (Bit Position and Value)

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Subnetting Chart (Subnet Mask Identifier)

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Subnetting

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Subnetting Chart

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Subnetting Example
• This is an example of subnetting the 192.168.10.0 class C
network into 8 subnets with 32 host addresses per subnet
• Note that the first and last subnets are not used (the first can be)
• Also the first and last host address in each subnet are not used

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Example Host IP Address from Subnet 2
Packet Address 192.168.10.65 11000000.10101000.00001010.010 00001
Subnet Mask 255.255.255.224 11111111.11111111.11111111.111 00000
Subnet Address 192.168.10.64 11000000.10101000.00001010.010 00000
• The subnet mask is ANDed with the packet address to
determine the subnet address - as shown in the next slides

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The Logical ANDing Process

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Calculating the Subnet ID

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Subnet Mask Defines the Number of Subnets

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Summary