Routing Fundamentals and Subnets
CCNA 1 v3 – Module 10
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Routed and Routing Protocols
Routing protocols should not be confused with routed protocols:
Routing protocols determine the paths that routed protocols
follow to their destinations.
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Examples: RIP, OSPF, IGRP, EIGRP, IS-IS, BGP
Routed protocols provide addressing and support at the
network layer.
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Examples: IP, IPX, Appletalk, DECnet
There are protocols that do not support Layer 3, and these are
classed as nonroutable protocols
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Example: NETBEUI
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IP
IP is the most widely used implementation of a hierarchical
network-addressing scheme.
 Connectionless
 Unreliable
 Best-effort delivery
These terms do not imply that the system does not work well,
but that IP leaves verification to upper layer protocols.
Routing protocols work with IP to find the most efficient route
for data.
IP determines the contents of the IP packet header, which
includes addressing and other control information.
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Two types of delivery services:
1.
Connectionless
2.
Connection-oriented
Most network services use a connectionless delivery system:
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Destination is not contacted before a packet is sent.
Packets can switch to different paths, and possibly arrive out of order.
Referred to as packet-switched processes.
Comparison: the postal system.
In connection-oriented systems:
 Connection is established between the sender and the recipient before
any data is transferred.
 All packets travel sequentially across the same physical or virtual circuit.
 Referred to as circuit-switched processes.
 Example: telephone system.
The Internet is a connectionless network in which all packet deliveries are
handled by IP. TCP adds Layer 4, connection-oriented reliability services.
Anatomy of an IP Packet
Header length - total length of all header
information, two variable-length header fields
Specifies level of importance assigned
by a particular upper-layer protocol
Length of packet, including
data and header
Version – IP version used;
must match receiving device
Sequence number
Specifies no. of hops
a packet may travel
Supports options
such as security,
variable length
Indicates upper-layer
protocol: TCP or UDP
Control fragmentation
Helps ensure IP header integrity
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IP Routing Protocols
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3 function.
Routing is an OSI Layer ___
most efficient path
Routing is the process of finding the _________________
from one device to another
router
The primary device that performs routing is the _______.
Two key functions of a router:
1.
Routers must maintain routing tables using a routing protocol
to communicate network information with other routers.
2.
The router switches the packets to the appropriate interface,
adds the necessary framing information for the interface, and
then transmits the frame.
Routing metrics are values used in determining the advantage
of one route over another.
The encapsulation and de-encapsulation process occurs each
time a packet transfers through a router.
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Packet Propagation and Switching within a Router
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Routing vs. Switching
 Routing and switching use different information in the process
of moving data from source to destination.
 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.
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ARP Tables and Routing Tables
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IGP and EGP
IGPs can be further categorized as Distance-Vector or Link State.
Examples of Distance-Vector protocols:
RIP, RIPv2, IGRP, (EIGRP)
Examples of Link-State protocols:
OSPF, IS-IS
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Subnetting
Why use Subnetting?
 Reduce network traffic
 Optimise network performance
 Simplify management
 Facilitate spanning of large geographical areas
The 32 bits of an IP address can be divided into three parts:
NETWORK-SUBNET-HOST
Example 1:
172.16.10.5 255.255.255.128
Can also be written:
172.16.10.5/25
Binary:
10101100.00010000.00001010.10000101
 Class B Network address  Subnet  Host 
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Values in a Subnet Mask
128
64
32
16
8
4
2
1
The Subnet is represented by an unbroken series of ‘1’s in the mask.
Only possible values (other than 0) for each octet in a subnet mask are:
128
192
224
240
248
252
254
255
Mechanics of Subnetting - ANDing
ANDing is a binary process by which the router calculates the subnetwork ID
for an incoming packet.
The IP address and the subnetwork address are ANDed with the result being
the subnetwork ID.
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Determining Subnet, Broadcast Address and Valid Host Range
Example:
Host: 172.16.10.33
Subnet Mask: 255.255.255.224
Is this a Class A, Class B or Class C address? Class B
Therefore how many bits make up the Network portion? 16
How many bits are in the Subnet Mask?
11111111.11111111.11111111.11100000 = 27
How many bits are left for Host addresses? 5
How many separate addresses in each subnet? 32
Which address represents the whole subnet? 172.16.10.32
Which address is used to broadcast to the subnet? 172.16.10.63
Which addresses are left in the valid host range?
First Valid: 172.16.10.33
Last Valid: 172.16.10.62
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Determining Subnet, Broadcast Address and Valid Host Range
Example 2:
Host: 192.168.100.25
Subnet Mask: 255.255.255.252
Is this a Class A, Class B or Class C address?
Therefore how many bits make up the Network portion?
How many bits are in the Subnet Mask?
How many bits are left for Host addresses?
How many separate addresses in each subnet?
Which address represents the whole subnet?
Which address is used to broadcast to the subnet?
Which addresses are left in the valid host range?
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