3. (a) Describe the purpose of having an IP address as well as a MAC address used

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3. (a) Describe the purpose of having an IP address as well as a MAC address used
in a network host. (b) Why are different inter
AS and intra
AS protocols used in the

: (a)
By having different kinds of address in network layer and link layer, we can
use different kinds of protocols (not
a 802.3

protocol) in our LAN side
connecting to the same WAN running in IP protocol.

Furthermore, since the IP
address is designed

in a hierarchical structure and most of the time location
dependent, whenever a host moves to a new location, it can register
the local
administrator for a new IP address rather than updating the IP tables around the
whole Internet.

(b) We use differen
t inter
AS and intra
AS routing protocols to solve the scalability
and administrative authority problems. The scalability problem means,
if we have all
our routers in the Internet running on a single routing protocol,
as the number of
routers become large,

the overhead involved in computing, storing, and
communicating routing information

will become unacceptable.
The administrative
authority problem means each ISP or organization may want to run its routers
whatever routing protocols it please or hide aspec
ts of its network

s internal
organization from the outside.

4. (a)
Use link
layer switches as an example to explain collision domains and
play. (b) What is the fundamental difference between a link
layer switch
and a router? (c) How to resolve th
e hidden terminal problem in wireless LANs?

Ans: (a)
Collision domain on a switch: each link connected to the switch is its own
collision domain. The switch will only forward frames that destined to that link.
play on a switch: when using a switch
, it does not need to be configured. It
has a self

that builds the switch table on its own.

(b) A switch is
a link
layer device which examines the link
layer header to forward frames, while a
router is a network
layer device that examine
s the
layer header to route


Using the RTS and CTS signals. When a sender wants to transmit its frame
to a receiver, it will first send a RTS to notify the receiver to reserve channel for it.
The receiver will broadcast a CTS to give th
e sender the permission to send while

other nodes

their frames to it for that reserved duration.

5. A student attaches a laptop to the NCTU campus network. He/she then requests
and receives
. Identify and briefly describe protocols(at all layers)
involved in requesting and receiving such a www page.


HTTP(application layer),

request and receive the content

of web pages;
In this case,
the page of


ation layer), query the IP address of a domain name
. In this case, query

the IP


TCP(transport layer), provide a reliable , full
duplex connection between two


In this case, your laptop may establish a TCP

connection to the server of
, then

requesting and receiving data on it.

UDP(transport layer), prov
ide an unreliable, connectionless, fast data transmissi

In this case, your laptop may send a HTTP request to the
www.google.com encapsulated in a

UDP datagram

and the
remote server will send a HTTP response back to your laptop
also encapsulated in a

UDP datagram.

RIP or OSPF or BGP(network layer),

AS or inter
AS routing protocols.

In this
case, it helps your packets correctly route
between routers from your local network to
the server of www.google.com.

(link layer), find the MAC address that corresponds to a known IP address.

this ca
se, when the first time you send a packet to the
www.google.com, your laptop send a

ARP request to ask which
MAC address corresponds
or serve

this IP address
. Since the
destination IP address is outside the LAN, the gateway router in
your LAN will res
pond such ARP with its MAC address. Then your
laptop will send the frame containing packet destined to

to the gateway router, and the router will route
the packet to the next hop toward destination.

EE 802.3(link layer), use CSMA/CD to forward frames in LAN.

In this case, it helps

your laptop communicate with routers or DNS server in your
LAN via 802.3 Ethernet protocol.

Others may i
nclude ICMP or

802.11 wireless