Cisco
Certified Network Associate CCNA 2.0
(640-801)
About CCNA
Exam: CCNA exam tests you in the areas of
simple LAN/WAN switching, Cisco IOS, and routing. Topics include
TCP/IP model of internetworking, configuring, and
troubleshooting some of the most widely used Cisco
switches and routers. Also, CCNA is foundation exam for
CCNP (Cisco Certified Networking Professional). The exam notes is a brief review
of important points that help in quick review of key
points.
1. Internetwork IP addressing:
IP addresses are written using decimal
numbers separated by decimal points. This is called
dotted decimal notation of expressing IP addresses.
The different classes of IP addresses is as
below:
|
Class
|
Format
|
Leading Bit
pattern
|
Network address Range
|
Maximum
networks
|
Maximum hosts/ nodes
|
|
A
|
N.H.H.H
|
0
|
0-126
|
127
|
16,777,214
|
|
B
|
N.N.H.H
|
10
|
128-191
|
16,384
|
65,534
|
|
C
|
N.N.N.H
|
110
|
192 -223
|
2,097,152
|
254
|
- Network address of all zeros means
"This network or segment".
- Network address of all 1s means " all
networks", same as hexadecimal of all Fs.
- Network number 127 is reserved for
loop-back tests.
-
Host (Node) address of all zeros mean "This Host (Node)".
- Host (Node) address of all 1s mean "all
Hosts (Nodes) " on the specified network.
2. The range of numbers from 224.0.0.0 to
239.255.255.255 are used for multicast packets. This is
known as Class D address range.
3. Subnetting is nothing but creating
networks within a network. Subnetting allows an
organization with a single IP address (Class A /ClassB
/ClassC)
to have multiple subnetworks, thus allowing several
physical networks within the organization.
4.
How to maximize the number of
subnets for a given number of hosts:
Let us take a
network ID of 168.8.0.0, and find the maximum number of
possible subnets and the corresponding subnet mask that
can accommodate at least 500 hosts. The steps involved
are outlined below:
I. Find the Class of the IP
address, in this case it is a class B network. Class B
network has the form N.N.H.H. Therefore, we have a total
of 16 bits (two octets) for assigning to internal
networks and hosts. The minimum number of host addresses
required is 500. The last octet corresponds to 2^8 = 256
hosts which is still less than 500 Hosts.. Therefore,
you have to borrow one more bit from the third octet to
make it 256*2 = 512 Hosts. This leaves 7 bits in the
third octet for assigning subnet addresses. This is
equal to 2^7=128 subnets.
II. Write the 7 bits
available for subnetting in third octet in the form
11111110 (last bit being the Host bit). The decimal
equivalent of the first seven bits is
2^7+2^6+2^5+2^4+2^3+2^2+2^1
=
128 + 64 +32 + 16 + 8 + 4
+ 2 =
254.
III.
Therefore, the subnet mask
required is 255.255.254.0.
5.
How to maximize the number of hosts for a given number of
subnets:
Determining the subnet mask that allows maximum
number of hosts:
Let us consider an IP address 196.202.56.0 with
four subnets and maximize the number of host for the
given subnets. The steps involved are as below:
I. The
number of subnets required are four. We need to
add subnets of all ones and all zeros to this. This is
because all zeros and all ones subnets belong to
"this subnet" and "all subnets"
broadcasts and can not be used. Therefore, the total
number of subnets to be reserved is 4+2 = 6.
II. We want to implement
maximum possible Hosts. Therefore, we need to minimize
the number of subnets. This minimum number is 6 here. If
we reserve 2 bits, it results in only 2^2=4 subnets
which is less
than 6. Therefore, we have to reserve 3 bits for implementing
subnets, resulting in 2^3=8 subnets. This is now
optimized for maximum number of Hosts (as we have
optimized for minimum number of subnets).
III. Write the 3 bits
available for subnetting in fourth octet in the form
11100000 (Five 0s being Host bits). The decimal
equivalent is 2^7+2^6+2^5
= 128 + 64
+32 =
224.
IV.
Therefore, the subnet mask required is 255.255.255.224.
6.
127.0.0.1 is the local loop back address.
7.
In an internetwork, the number of distinct IPs' required are
-
One each per client
computer
-
One each per server
computer
-
One each per router
interface.
For
example, your network has 2 servers, 26 clients
machines, and 2 router interfaces the total number of IP
addresses required are 30.
8.
Finding the number of Hosts and subnets
available for a given subnet mask: For example, let us
find the number of hosts and subnets available for an IP
156.233.42.56 with a subnet mask of 7 bits.
a. Class B network has the
form N.N.H.H, the default subnet mask is 16 bits long.
There is additional subnet mask of 7 bits long.
b. 7 bits of subnet
mask corresponds to (2^7-2)=128-2 = 126 subnets.
c. 9 bits (16-7) of
host addresses corresponds to (2^9-2)=512-2 = 510 hosts.
Some times, the
subnet mask is specified with the bits available in the
default subnet mask. In this case the bits available in
default subnet mask is 16. Therefore, total number of
bits available in the subnet mask are 16+7=23. If you
are given a subnet mask of 23 bits long for a class B
address, it is understood that it contains the bits from
the default subnet mask as well.
Hence, 126 subnets
and 510 hosts are available.
9.
The directed broadcast
should reach all Hosts on the intended network (or
subnet, if sub netted). For example, the directed
broadcast address for an IP network 196.233.24.15 with
default subnet mask is 196.233.24.255. This is arrived
by putting all 1s for the host potion of the IP address.
10.
Telnet, FTP, and TFTP:
-
Telnet is used for terminal emulation that runs
programs remotely. Telnet uses TCP/IP protocol.
-
Telnet requires a
username and password to access.
-
FTP
(File Transfer Protocol) is a connection oriented
protocol. It uses TCP/IP for file transfer. Compare this
with TFTP (Trivial File Transfer Protocol) that uses UDP
(Connectionless protocol). SNMP uses UDP over IP.
Tracert, Ping use ICMP as their base protocol. FTP is used to transfer files.
-
Both
FTP and Telnet are client-server protocols. Note that
TCP/IP is a client server oriented protocol.
11.
Maximum hop count
supported by RIP is 15.
12. The port numbers used
by different programs are as below:
I.
FTP : Port #21
II.
Telnet: Port #23
III.
SMTP: Port #25
IV.
SNMP: Port #161
It is important to know that FTP,
Telnet, SMTP use TCP; whereas TFTP, SNMP use UDP.
13.
SNMP is part of
TCP/IP protocol suite. It allows you to monitor and
manage a network from a centralized place by using SNMP
Manager software. The systems or devices that provide
the responses are called agents (or MIBs). An SNMP agent
is any computer running SNMP agent software.
MIB stands for Management
Information Base. It is part of SNMP agent database. A
MIB records and stores information abut the host it is
running on. An SNMP manager can request and collect
information from an agent's MIB. Routers are typical MIB
agents. SNMP agent generates "trap" messages
that are then sent to an SNMP management console, which
is a trap destination.
14.
Address
Resolution Protocol (ARP) is used to resolve or map a
known IP address to a MAC sub layer address to allow
communication on a multi-access medium such as Ethernet.
Reverse ARP (RARP) is used to obtain an IP address using
an RARP broadcast. RARP can be used to boot diskless
workstations over a network.
15.
The 7 layers of OSI
model are:
1.
The Application Layer: Application layer is responsible
for identifying and establishing the availability of
desired communication partner and verifying sufficient
resources exist for communication. Some of the important
application layer protocols are: WWW, SMTP, FTP, etc.
2.
The Presentation Layer: This layer is responsible for
presenting the data in standard formats. This layer is
responsible for data compression, decompression,
encryption, and decryption. Some Presentation Layer
standards are: JPEG, MPEG, MIDI, PICT, Quick Time, TIFF.
3.
The Session Layer: Session Layer is responsible for co-ordinating
communication between systems/nodes. The following are some of the session layer protocols and
interfaces: a) Network File System (NFS), SQL, RPC (Remote
Procedure Call), X-Windows, ASP, DNA SCP.
4.
The Transport Layer: The Transport Layer is responsible
for multiplexing upper-layer applications, session
establishment, and tearing-down of virtual circuits.
This layer is responsible for flow control, to maintain
data integrity.
5.
The Network Layer: There can be several paths to send a
packet from a given source to a destination. The primary
responsibility of Network layer is to send packets from
the source network to the destination network using a
pre-determined routing methods. Routers work at Network layer.
6.
The Data Link Layer:
Data
Link Layer is layer 2 of OSI reference model. This layer
is divided into two sub-layers:
A.
Logical Link Control (LLC) sub-layer.
B.
Media Access Control (MAC) sub-layer.
The LLC
sub-layer handles error control, flow control, framing,
and MAC sub-layer addressing.
The MAC
sub-layer is the lower of the two sub-layers of the Data
Link layer. MAC sub-layer handles access to shared
media, such a Token passing or Ethernet.
7.
Physical Layer: The actual flow of signals take place
through Physical layer. At Physical layer, the interface
between the DTE and DCE is determined. The following are
some of the standard interfaces are defined at Physical
layer: EIA/TIA-232, EIA/TIA-449,V.24,V.35,X.21,G.703,HSSI
(High Speed Serial Interface).
16.
Repeaters, Bridges, and Routers:
I.
Repeaters work at Physical layer (Layer 1),
II. Bridges and simple switches work at Data Link Layer
(Layer 2),
III. Routers work at Network Layer (Layer 3) of ISO Reference
Model.
17.
CSU / DSU is an acronym for
Channel Service Unit / Data Service Unit. CSU/DSU is
part of Customer Premise Equipment (CPE). CSU / DSU
connect to a Central Office (CO), a telephone switching
company located nearer to the customer.
18.
Spanning Tree Protocol (STP)
IEEE Specification 802.1d is used to prevent
routing loops. In Cisco Catalyst 5000 series switches,
use BDPUs (Bridge Protocol Data Units) to determine the
spanning tree topology. STP uses a Tree Algorithm (STA)
to prevent loops, resulting in a stable network
topology.
19.
HTTP is the protocol used for
accessing the World Wide Web services. HTTP operates
over TCP/IP. TCP/IP is the protocol, which is used by
all internet applications such as WWW, FTP, Telnet etc.
IPX/SPX is proprietary protocol stack of Novell
NetWare.
20.
1.
The term "Segments" is usually associated with Transport layer
2.
The term "Packets" is usually associated with Network Layer and
3.
The term "Frames" is usually associated with Data Link Layer
Any comments or
feedback on the above notes may please be directed to the contributor here
