04 - IP Addressing and Routing

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04 IP Addressing & Routing

By Muhammad Asghar Khan

Reference: CCENT/CCNA ICND1 Official Exam Certification Guide By Wendell Odom


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Agenda

Introduction Network Layer (L3)

Logical Addressing

IP Addressing Network Classes IP Subnetting

Routing / Forwarding

Host Routing

Router Routing Routing Protocols

Network Layer Utilities

DNS, ARP, DHCP, Ping2 www.asghars.blogspot.com

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Introduction

www.asghars.blogspot.com3

The OSI physical layer (L1) defines how to transmit bits overa particular type of physical network

The OSI data link layer (L2) defines the framing, addressing,error detection, and rules for when to use the physicalmedium

The OSI network layer (L3) defines how to forward, or route,data between the two computers i.e end-to-end delivery ofdata between two computers

OSI network layer protocol defines the following features:

Logical Addressing (IP Addressing)

Routing (IP Routing)

Routing Protocols

Network Layer Utilities (DNS, ARP, DHCP and Ping)

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Logical Addressing


Network layer protocols define the format andmeaning of logical addresses/network layer

addresses

Each computer (NIC) that needs to communicate

will have at least one logical address

IP is an example of network layer address

Logical address allows for the logical grouping of

addresses, with IP this is called subnet or network

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IP Addressing


If a device wants to communicate using TCP/IP, itneeds a unique IP address

Any device that can send and receive IP packets iscalled an IP Host

IP address consists of a 32-bit number written indotted-decimal notation e.g. 172.16.254.1

Each decimal number in IP address

is called an octet The range in each decimal number

is b/w 0 and 255

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IP Addressing


The set of consecutive addresses grouped togetheris called IP Networks

Grouping make routing easy

Figure shows the concept of IP address grouping

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IP Addressing


The figure on last slide summarize the two main

facts about same grouping:

All IP addresses in the same group must not be

separated by a router

IP addresses separated by a router must be indifferent groups

IP defines five different network classes

Unicast Addresses: Class A, Class B, Class C Multicast Addresses: Class D

Experimental Addresses: Class E

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IP Addressing


In class A, B and C the IP address can be divided intwo parts

Network Portion; identifies the number of networks

Host Portion; identifies the number of hosts on the

LAN

IP Address

(130.4.0.1)

Network Portion(130.4)

Host Portion(0.1)

e.g. Zip code

of address

e.g. Street address

with in zip code

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IP Addressing


Class A, B and C networks each have differentlength for the part that identifies the network

Class A Networks

First octet range is from 1 to 126

1 byte long network part while remaining 3 bytes of

address is called host part, e.g N.H.H.H

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0 XXXXXXX . XXXXXXXX . XXXXXXXX .

XXXXXXXX

0 identifies Class A

7-Bits represent network part 24-Bits reserved for host part


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IP Addressing


Class B Networks First octet range is from 128 to 191

2 bytes long network part while remaining 2 bytes of

address is called host part, e.g N.N.H.H

10 XXXXXX . XXXXXXXX . XXXXXXXX .

XXXXXXXX

10 identifies Class B

14-Bits represent network part 16-Bits reserved for host

part

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IP Addressing


Class C Networks First octet range is from 192 to 223

3 bytes long network part while remaining 1 byte of

address is called host part, e.g N.N.N.H

110 XXXXX . XXXXXXXX . XXXXXXXX .

XXXXXXXX

110 identifies Class C

21-Bits represent network part 8-Bits reserved for host

part

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IP Addressing


Class A is for large networks, Class B is for mediumnetworks and Class C for small networks

We must distinguish b/w network number and IP

address

Network numbers are not actually IP addresses

because they cannot be assigned to an interface as

an IP address

For network numbers the convention is to write

down the network part of the number, while all

decimal 0s in the host part of the number

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IP Addressing


For example; Class B network 130.4 will consistsof all IP addresses that begin with 130.4, written as

130.4.0.0 and so on

*There are two reserved host addresses per network. These are network numbeNetwork/directed broadcast address (one with all binary 1s in the host part).

Network number is the lowest numerical value inside that network and the broad

address is the largest.

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IP Addressing


Internet Corporation for Assigned NetworkNumbers (ICANN) is in charge of assigning the IP

addresses

Table below summarizes the possible network

numbers that ICANN have assigned over a time

*The Valid Network Numbers column shows actual network numbers.

Networks 0.0.0.0 (originally defined for use as a broadcast address not now)

and 127.0.0.0 (still available for use as the loopback address) are reserved.

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IP Addressing


IP Subnetting IP Subnetting takes a single Class A, B or C network

and subdivides it into a number of smaller groups of

IP addresses

Subnetting treats a subdivision of a single Class A, Bor C network as if it were a network itself

Subnet is shorthand for subdivided network

Figure on next slide shows the network withoutsubnetting

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IP Addressing


1

2

34

5

6

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IP Addressing


The design on previous slide requires six groups of IPaddresses, each of which is a Class B network

Each Class B network has 216-2 host addresses-far morethan you will ever need for each LAN and WAN link

For example; the upper-left Ethernet should contain all

addresses that begin with 150.1. Therefore, addressesthat begin with 150.1 cannot be assigned anywhere elsein the network, except on the upper-left Ethernet

So, if you ran out of IP addresses somewhere else, you

could not use the large number of unused addresses thatbegin with 150.1. As a result, the addressing designshown in previous slide wastes a lot of addresses

This design would not be allowed if it were connected tothe Internet, ICANN would not assign six Class B network

numbers

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IP Addressing

18

subnet1 subnet2

subnet3

subnet4

subnet5

subnet

6

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IP Addressing


This design subdivides the Class B network150.150.0.0 into six subnets

The third octet is used to identify unique subnets of

network 150.150.0.0

The subnet part of the address is created byborrowing bits from the host part of the address

The host part of the address shrinks to make room for

the subnet part of the address

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IP Addressing


Figure on previous slide shows the format of addresses

when subnetting, representing the number of bits in each

of the three parts of an IP address

Now, instead of routing based on the network part of an

address, routers can route based on the combined

network and subnet parts

Note that the information in the routing table includes

both the network and subnet part of the address,

because both parts together identify the group

In figure on previous slide, there are three parts of an IP

address (network, subnet, and host), are called classful

addressing

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IP Addressing


Classfull Addressing: Net Part+Subnet+Host Part Classless Addressing: Routing Part

(Network+Subnet)+Host Part

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Routing focuses on the end-to-end logic offorwarding data

The routing process forwards only the packetdiscarding data link headers and trailers along the

way Host Routing/Host Forwarding

The host routing logic is a two-step process:

If destination IP address is in the same subnet as I am,

send packet directly

If destination IP address is not in the same subnet as Iam, send the packet to my default gateway (routersEthernet interface on the subnet)

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Router Routing/Router Forwarding A router uses the following logic when receiving data

link frame (frame with IP packet encapsulated)

1) Uses the data link FCS field to ensure that frame had

no errors; if errors occurred discard the frame2) If no errors occurred, discard the data link header and

trailer, leaving the IP packet

3) Compare the IP packets destination IP address to the

routing table, and find the route that matches thedestination address. This route can be outgoing

interface of the router and possibly the next-hop

router

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4) Encapsulate IP packet inside a new data link header

and trailer, and forward the frame

Figure on next slide shows simple routing example

with IP subnets

Step A: PC1 builds IP packet, with destination addressof PC2s. PC1 sends the packet to R1 (default

gateway) bcz destination address is on a different

subnet . PC1 places IP packet into Ethernet frame,

with a destination Ethernet address of R1s Ethernet

address

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Step B: R1 copies the frame off the Ethernet , checksframes FCS, discards the Ethernet header & trailer. R1compares the packets destination address to the routingtable and forward the packet to next-hop router R2

Step C:R2 receives the HDLC frame, check frames FCS

field, discard HDLC header & trailer. R2 finds route forsubnet 150.154.4.0 and sends the packet out interfaceserial1 to next-hop router R3

Step D: R3 also check FCS field, discard data link header &trailer. Checks its own route for subnet 150.150.4.0, as R3is directly connected to subnet 150.150.4.0, so it has toencapsulate the packet inside Ethernet header & trailerwith destination Ethernet address of PC2s MAC addressand forward the frame

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Routing Protocols


IP routing protocols fill the routers routing table withloop-free routes

Routing / Forwarding process depends on accurate andup-to-date IP routing table on each router

Each route includes a subnet number, interface out toforward packets and IP address of the next router ifneeded

Goals of IP routing protocol:

To dynamically learn and fill routing table with a route toall subnets

If more than one route to a subnet is available, place thebest route in the routing table

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Routing Protocols


To remove the non valid route from routing table

To add new routes or to replace lost routes, as quickly aspossible. The time b/w losing the route and finding aworking replacement route is called convergence time

To prevent routing loops

Routing protocols use the following logic:

Step 1: Each router adds a route to its routing table foreach subnet directly connected to the router

Step 2: Each router tells its neighbors about all the routes

in its routing table Step 3: After learning a new route from a neighbor, the

router adds a route to its routing table. Neighbor istypically the next-hop router

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Routing Protocols


Figure showshow the router R1

learns about

subnet150.150.4.0

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Routing Protocols


Step A: R3 learns a route that refers to its own E0interface

Step B: R3 sends a routing protocol message calledrouting update to R2, causing R2 to learn about

subnet 150.150.4.0 Step C: R2 sends a similar routing update to R1,

causing R1 to learn about subnet 150.150.4.0

Step D: R1s route to 150.150.4.0 lists 150.150.2.7(R2s IP address) as the next-hop address becauseR1 learned about the route from R2. The route alsolists R1s outgoing interface as Serial0

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The four network layer utilities i.e DNS, ARP, DHCPand Ping are used to help the network layer inrouting packets from end-to-end through aninternetwork

Domain Name System (DNS) DNS translate the friendly "www.google.com" to the

not-so-friendly 64.17.143.84

It handles this translation for web sites, email, FTP

servers, database servers, or any machine within adomain name

Figure on next slide shows the DNS request and reply

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Hannah knows the IP address of a DNS server, bcz the

address was either preconfigured or was learned

from DHCP

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Address Resolution Protocol (ARP)

ARP resolve an IP address to the MAC address

Figure below shows simple ARP process

An ARP broadcast is sent to a broadcast Ethernet address,so everyone on the LAN receives it

Because Jessies IP address is 10.1.1.2 and the ARPbroadcast is looking for the MAC address associated with10.1.1.2, Jessie replies with her own MAC address

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Now Hannah knows the destination IP and Ethernet

addresses that she should use when sending frames toJessie, and the packet shown in Figure can be sentsuccessfully

If Hannah and Jessie had been in different subnets,Hannahs routing logic would have caused Hannah to

want to send the packet to Hannahs default gateway(router). In that case, Hannah would have used ARP tofind the routers MAC address instead of Jessies MACaddress

Hosts need to use ARP to find MAC addresses only once in

a while. Any device that uses IP should retain, or cache,the information learned with ARP, placing the informationin its ARP cache

You can see the contents of the ARP cache by using arp -a

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Dynamic Host Configuration Protocol (DHCP)

DHCP allow computers to request a lease of an IPaddress

DHCP uses a server, that keeps a list of pools of IP

addresses available in each subnet DHCP client send the DHCP server a message, asking

to borrow or lease an IP address

DHCP also supplies subnet mask , default gateway as

well as IP address of any DNS servers Figure on next slide shows typical set of messages

used b/w a DHCP server to assign IP, as well as the IPaddress(es)

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Ping Command

After a network is implemented, ping (Packet InternetGroper) is the primary tool for testing basic network

connectivity

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Ping uses the Internet Control Message Protocol

(ICMP), sending message called an ICMP echo reply

ICMP does not rely on any application, so just tests

basic IP connectivity Layers 1, 2 and 3

Figure below outlines the basic process

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04 - IP Addressing and Routing

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