Part-1 - NOVA Student Web

gazecummingNetworking and Communications

Oct 26, 2013 (4 years and 8 months ago)




What does the transport layer do?

The transport layer performs three functions:

Establishing end
end connections: linking the application layer to the

Addressing: finding the address of the ultimate destination computer.

Packetizing: bre
aking long messages into smaller packets for transmission.


What does the network layer do?

The network layer performs two functions:

Routing: determining the next computer to which the message should be
sent to reach the final destination

Addressing: fin
ding the address of that next computer.


What are the parts of TCP/IP and what do they do? Who is the primary user of

As the name implies, TCP/IP has tow parts. TCP is the transport layer

protocol that links the application layer to the network laye
r. It performs
packetizing: breaking the data into smaller packets, numbering them, ensuring
each packet is reliably delivered, and putting them in the proper order at the
destination. IP is the network layer protocol and performs addressing and routing.
P software used at each of the intervening computers through which the message
passes; it is IP that routes the message to the final destination.

The TCP/IP was developed for the U .S. Department of Defense’s
Advanced Research Project Agency network (ARP
ANET). TCP/IP is the
transport/network layer protocol used on the Internet. It is also the world’s
popular network layer protocol, used by almost 80 percent of all BNs, MANs. In
1998, TCP/IP moved past IPX/SPX as the most common protocol used on LANs.

6. W
hy is TCP/IP the most popular protocol?

TCP/IP is the world’s popular network layer protocol, used by
almost 80 percent of all BNs, MANs. In 1998, TCP/IP moved past IPX/SPX as
the most common protocol used on LANs. TCP/IP allows reasonably efficient and
free transmission. Because it performs error checking, it can send large files
across sometimes
unreliable networks with great assurance that the data will
arrive uncorrupted. TCP/IP is compatible with a variety of data link protocols,
which is one r
eason for its popularity.

7. Compare and contrast the three types of addresses used in a network.

Computers can have three different addresses: application layer
address, network layer address, and data link layer address. Data link layer
addresses are u
sually part of the hardware whereas network layer and application
layer addresses are set by software. Internet registrars assign network layer and
application layer addresses for the Internet. Addresses within one organization are
usually with the same fi
rst 3 bytes. Subnet masks are used to indicate whether the
first 2 or 3 bytes (or partial bytes) indicate the same subnet. Some networks assign
network layer addresses in a configuration file on the client computer whereas
others use dynamic addressing in
which a DHCP server assigns when a computer
first joins the network.

8. How is TCP different from UDP?

TCP/IP can operate either as connection
oriented or connectionless. When
oriented is desired, both TCP/IP are used. TCP establishes the virtua
circuit with the destination and informs IP to route all messages along this virtual
circuit with the destination and informs IP to route all messages along this virtual
circuit. When connectionless is desired, the TCP packet is replaced with a User
gram protocol (UDP) packets. The UDP is much smaller than the source port,
destination port, message length, and checksum.

oriented routing has greater overhead than connectionless
routing, because the sender must first “open” the circuit by se
nding a control
packet that instructs all the intervening devices to establish the circuit routing.
Likewise, when the transmission is complete, the sender must “close” the circuit.
oriented protocols also tend to have more overhead bits in each


Connectionless routing means each packet is treated separately and makes
its own way through the network. It is possible that different packets will take
different routes through the network. It is possible that different packets will take
ent routes through the network depending upon the type of routing used and
the amount of traffic. Because packets following different routes may travel at
different speeds, they may arrive out of sequence at their destination. The sender’s
network layer th
erefore puts a sequence number on each packet, in addition to
information about the message stream to which the packet belongs. The network
layer must reassemble them in the correct order before passing the message to the
application layer.

9. How does TCP

establish a connection?

TCP sets up a virtual circuit between the sender and the receiver. The
transport layer software sends a special packet (called a SYN, or synchronization
characters) to the receiver requesting that a connection be established. The
receiver either accepts or rejects the connection, and together, they settle on the
packet sizes the connection will use. Once the connection is established, the
packets flow between the sender and the receiver, following the same route
through the network

10. What is a subnet and why do networks need them?

Each organization must assign the IP addresses it has received to specific
computers on its networks. In general, IP addresses are assigned so that all
computers on the same LAN have similar address.

bnet in other words can be
described as a group of computers on the same LAN with IP numbers with the
same prefix. For

example, suppose a university has just received a set of Class B
addresses starting with 128.184.x.x. It

customary to assign all the
computers in the same LAN numbers that starts with
the same first three digits, so the Business School LAN might be assigned
128.184.55.x. Likewise, all the other LAN at the university and the backbone
network that connects
them would have a different set
of numbers.


of these
LANs is called a TCP/IP subnet because IP number logically groups computers in
LAN. Knowing a computer is on your subnet is very important for message

11. What is a subnet mask?

Subnet masks tell computers what part of
an Internet protocol (IP) address
is on the same subnet or on a different subnet. A subnet mask is a 4
byte binary
number that has the same format as an IP address. A1 is the subnet mask indicates
the subnet. A0 indicates that it is not.

12. How does dynam
ic addressing work?

With dynamic addressing
, a s
erver is designated to supply a network

address to a computer each time the computer connects to the network. This is
commonly done for client computers but usually not done for servers.

Instead of prov
iding a network layer address in a configuration file, a
special software package is installed on the client that instructs it to contact bootp
or DHCP severs using data link layer addresses. This message asks the servers to
assign the requesting computer
a unique network layer address. The server runs a
corresponding bootp or DHCP software package that responds to these request
and sends a message back to the client giving its network layer address.

13. What benefits and problems does dynamic addressing p

The bootp or DHCP server can be configured to assign the network

Layer address to the computer each time it requests an address, or it can lease the
address to the computer by picking in the “next available”

network layer address
from a list of aut
horized addresses. Addresses can be leased for as long as the
computer is connected to the network for a specified time limit. When the lease
expires, the client computers must contact the bootp or DHCP server to get a new
address. Address leasing is commo
nly used by Internet service provider (ISPs) for
up users.

Dynamic addressing greatly s
implifies network management in

p networks.

With dynamic addressing, address changes need to be
made only to the bootp or DHCP server, not to each indivi
dual computer. The
next time each computer connects to the network or whenever the address lease
expires, the computer automatically gets the new address.

14. What is address resolution?

Address resolution is the process of translating an application laye
address into a network layer address or translating a network layer address into
data link layer address.

15. How does TCP/IP perform address resolution for network layer addresses?

Server name resolution is the translation of application layer addresse
into network layer addresses (e.g., translating an Internet address such as

into an IP address such as This is done using the
Domain Name Service (DNS). Throughout the Internet there are

a series of
computers called name servers that provide DNS services. These name servers
run special address database that store thousand of Internet address and their

IP address. These name servers are in effect the “directory
assistance” co
mputers for the Internet. Any time a computer does not know the IP
number for a

it sends a message to the
name server requesting the IP

When TCP/IP needs to translate an application layer address into an IP
address, it sends a special TC
level packet to the nearest DNS server. This
packet asks the DNS server to send the requesting computer the IP address that
matches the Internet address provided. If the DNS server has a matching name in
its database, it sends back a special TCP packet w
ith the correct IP address. If that
DNS server has a matching name in its database, it sends back a special
packet with the correct IP address. If that DNS server does not have that Internet
address in its database, it will issue the same request to an
other DNS server
elsewhere on the Internet.

Once your computer receives an IP address it is stored in a server address
table. This way, if you ever need to access the same computer again, your
computer does not need to contact a DNS server. Most server add
ress tables are
routinely deleted whenever you turn off your computer.

. How does TCP/IP perform address resolution for data link layer addresses?

To send a message to another computer in its subnet, a computer must
know the correct data link layer addr
ess. In this case, the TCP/IP software sends a
broadcast message to all computers in its subnet. A broad cast message, as the
name suggests, is received and processed by all computers in the same LAN
. The
message is specially formatted request using Addres
s Resolution Protocol (ARP)
that says, “who ever is IP address, please send me your data link
layer address.” The software in the computer with that IP address then sends an
ARP response with its data link layer address. The sender transmit
s its message
using that data link layer address.

17. What is routing?

Routing is

the process of selecting the route or path through the network
that a message will travel from sending computer to the receiving computer.

18. How does decentralized routi
ng differ from centralized routing?

In centralized routing decisions are made by one computer and they are
used on small, mainframe
based networks. Where as in decentralized routing,
decisions are made by each node independently of one another and Informat

to be exchanged to prepare routing tables

and they were used by the

19. What are the differences between connectionless and connection

Connectionless routing means each packet is treated separately and makes
its own
way through the network. It is possible that different packets will take
different routes through the network depending on the type of routing used and
the amount of traffic.

oriented routing sets up a virtual circuit between the sender
and rec
eiver. The network layer makes one routing decision when the connection
is established
, and all packets follow the same route. All packets in the same
message arrive at the destination in the same order in which they were sent.

20. What is a virtual circui

A virtual circuit is a temporary transmission circuit in which sequential
data packets are routed between two points. It is created by the software in such a
way that user think they have a dedicated point
point leased circuit.

21. What is QoS routi
ng and why is it useful?

Quality of service routing is a special type of connection
oriented routing.
It is the ability of devices to give different priorities to different types of messages
so that some messages (e.g., voice telephone data) are transmitt
ed faster than
other messages (e.g., e

22. Compare and contrast unicast, broadcast, and multicast messages.

The most common type of message in a network is the transmission
between two computers. One computer sends a message to another computer
g., a client requesting a web page). This is called a unicast message. Broadcast
message is sent to all computers on a specific LAN or subnet. A third type of
message called a multicast message is used to send the same message to a group
of computers.

cast: This would work but would require lot of network capacity.

Broadcast: This would reduce network traffic (because each computer would
send one broadcast message), but every computer on the network would process
it, distracting from other tasks. Broa
dcast messages usually are transmitted only
within the same LAN or subnet, so this would not work if one of the computers
were outside the subnet.

This reduces the problem which appeared in unicast and broadcast.

23. Explain how multicasting


Computers wishing to participate in a multicast send a message to the
sending computer or some other computer performing routing along the way
using a special type of packet called Internet Group Management Protocol
(IGMP). Each multicast group is

assigned a special IP address to identify the
group. Any computer performing routing knows to route all multicast messages
with this IP address onto the subnet that contains the requesting computer. The
routing computer sets the data link layer address on

multicast messages to a
matching multicast
data link layer address. Each requesting computer must inform
its data link layer software to process incoming messages with this multicast data
link layer address. When the multicast session ends (e.g., the vide
oconference is
over), the client computer sends another IGMP message to the organizing
computer or the computer performing routing to remove it from the multicast

24. Explain how the client computer in Figure 5.14 (128.192.98.xx) would obtain
the da
ta link layer address of its subnet gateway.

When a computer is installed on a TCP/IP network (or dials into a TCP/IP
network), it knows the IP address of its subnet gateway. This information can be
provided by a configuration file or via a bootp or DHCP
server. However, the
computer does not know the subnet gateway’s Ethernet address (data link layer
address). Therefore, TCP would broadcast an ARP request to all computers on its
subnet, requesting that the computer whose IP address is to resp
with its Ethernet address.

All computers on the subnet would process this request, but only the
subnet gateway would respond with an ARP packet giving its Ethernet address.
The network layer software on the client would then store this address in its
link layer address table.

25. Why does HTTP use TCP and DNS use UDP?

HTTP at the application layer would pass its message packet with
overhead, including the Internet address of the destination, to the transport layer
where TCP software would complet
e packetization at the Transport layer and
hand it off to the Network layer.

Domain Name Services (DNS) is primarily responsible for translating IP
addresses into valid domain names and translating valid domain names into IP
addresses. UDP is a connection
less transport layer protocol.DNS would pass
either a domain name or an IP address along with its packet(s) to the Transport
layer for forwarding to the Network layer to be routed to the destination.

26. How does static routing differ from dynamic routing
? When would you use
static routing? When would you use dynamic routing?

With static routing, the routing table is developed by the network manager
and remains unchanged until the network manager updates it. With dynamic
routing, the goal is to improve ne
twork performance by routing message over the
fastest possible route; an initial routing table is developed by the network manager
but is continuously updated to reflect changing network conditions, such as
message traffic.

Static routing is used on rela
tively simple networks with few routing
options that rarely change.
For example, most LANs are connected to the
backbone, so static routing is
used. Whereas

the other uses routing tables at each
node that are updated dynamically , based on routing conditio
ns information
exchanged between routing devices.

27. What type of routing does a TCP/IP client use? What type of routing does a
TCP/IP gateway use? Explain.

The TCP/IP client uses static routing because the client must always point
to a single gateway ro

The TCP/IP gateway router uses dynamic routing because typically it must
process multiple requests for routing beyond the single segment that it physically

28. Why would a network manager want to have only TCP/IP as the transport and
ork layer protocols?

Many different standard transport and network protocols exist to perform
addressing, routing and packetizing. All provide formal definitions for how
addressing and routing are to be executed and specify packet structures to transfer
his information between computers. TCP/IP
allows reasonably efficient and
free transmission. Because it performs error checking, it can send large files
across sometimes
unreliable networks with great assurance that the data will
arrive uncorrupted.
TCP/IP is compatible with a variety of data link protocols.
This is one reason for its popularity.



3. Use the web to explor
e the differences between BOOTP
and DHCP. Which
is likely to become more popular? Why?

DHCP (Dynamic Host Configuration

Protocol) is an extension of BOOTP
(Bootstrap Protocol), the previous IP allocation specification. DHCP is based on
BOOTP and maintains some backward compatibility. The main difference is that
BOOTP was designed for manual pre
configuration of the host in
formation in a
server database, while DHCP allows for dynamic allocation of network addresses
and configurations to newly attached hosts. Additionally, DHCP allows for

So, existing BOOTP devices can communicate with DHCP servers and allow
DHCP requests to
cross routers running BOOTP forwarders. This level of
backward compatibility makes it easy for administrators to upgrade their network
devices from BOOTP to DHCP as needed, without having to replace all of the
clients at once or having to upgrade all of th
e routers. Several major
advancements beyond the BOOTP specifications provide significant advantages.
For example, DHCP supports the concept of a “lease” whereby a server can
allocate an address to a client for a specific amount of time. If you have more
evices than IP addresses, using shorter leases can help to keep you from running
out of addresses. If you have more addresses than devices, you can utilize
permanent leases or you can assign fixed addresses to specific devices similar to
BOOTP’S mechanism.

Also, DHCP incorporates a much more robust dialogue during lease
negotiation. Since the address can be assigned to the devices on an ad
hoc bases,
mechanisms need to be incorporated into the assignment procedure that allow for
a bro
der range of
s, as well as for a broader range of error handling
conditions. BOOTP protocol only allowed for two types of messages (request and
reply), while DHCP has seven possible message types that can be used during the
address assignment sequence.


5. Suppos
e a client computer ( in building B in Figure 5.14
requests a large web page from the server in building A (www. 1.
Assume that the client computer has just been turned on and does not know any
address other than those in its con
figuration tables. Assume that all gateways and
web servers know all network layer and data link layer addresses.

Part 3 (supply screen shots)

Screen shot of IPCONFIG

Screen shot of PING

Screen shot of ARP

Screen shot of TRACERT