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sushmi.J | Cisco CCNA | Module 4 - Layer 3 Network

By: sushmi.J | Related Course: Cisco CCNA | Published: June 20, 2016 | Modified: June 21, 2016
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ARP(address reolution protocol)

packet at layer 3 looks like. dest.ip,source.ip,data,FCS

if host one wants to ping host 2 the

host 1 need to create a ethernet link to send a data over host 2. as host 1 dont know host 2 MAC address so it sends a broadcast saying whoever has the IP the pls send the MAC address.the host 2 replyies with its mac address which is unicast transformation.ARC basically resolves MAC address and stores in the table.host 1 encapsulate the ip address in the frame in data field where the packet is present.host 2 decapsulates it.


Notepadbringing up serial connections

serial point to point link

routers are seldom connected back to back using CSUDSU will connecct to cloud

functions of CSUDSU:

1)clocking(physical bit transfer rate)

2)a data connecting equipment device

sh ip interfacebrief command has

methos-layer 1

protocol-layer 2

down down means-layer 1 issue

up up -layer 2 issue(it becos no keepaline,mismatch encapsulation,no clock rate set on DCE end)






Notepaddata transfer example .

router 1 eg:150/101.45.0/29

show interface fastethernet 0/0

test connection

ping which is broadcast .so it should go to everybody

show ip interface brief

config t

interface fastethernet 0/0

to assign an ip add

ip address


no shutdown

do show ip interface brief

u ll see ”up ,UP

do wr (to save)

router 2

no ip domain look

host r2

line console 0

loog synch

no exec-timeout

int f0/0

id address

router 3

do the same

go to router 1

using ping

ping ,.3,.4

the first ping failed becos it is doing ARP address.

show ip arp

clear arp-cache

debug ip icmp







Notepadexmaple problems



Notepadlayer 3

controlls communication between broadcast domain.if u want to go to one broadcast domain to another u need network layer

need a router to connect.

device works at layer 3 is router

funtion is sending data

protocol unit is packet for layer 3

address are used is IP address

ip address is 32 bit in length,written in 4 field of 8 bits each seperating a dor (dotted decimal format)

x.x.x.x-32 bit

each x is 8 bit

one x: can vary from 0-255


Class full IP addressing(old method):

first octect-1-126(ie the first x)

    the subnet mask no.sjould be 255 0 0 0

    CIDAR= /8( no of bits on in subnet mask)

Second octect-128-191

   subnet mask -255 255 0 0

   cidar notation-/16


octect 3-192-223

   subnet mask-



octect 4=224-239(multitasking)

octect 5=240-255(experimental)

no. is reserved for testing is called the local loop back(if u can reach the address or test for this address then that means the TCP/IP protocol is properly installed)




Notepadproblem solving.

The new method :Classless IP Addressing.

how can i get 30 i address for a company



Subnet mak:

ip address has network portion and host portion.the subnet mask separates both .



If a corresponding  bit in MASK is ON (ie)1 then it belongs to network. And if it is OFF then it belongs to host.

the netowrk address

the host address

the first valid address eg. (,….

the last valid address eg-

the broadcast -eg.


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