December 1998
NI222: NETWORKS AND THE INTERNET

QUESTION 2

Total Marks: 20 Marks

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Solutions and allocated marks are indicated in green.
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(a) Explain two advantages and two disadvantages of using a layered architecture in organising network protocols. [4]
Any two advantages, and two disadvantages, such as:

Advantages: Handle complexity (1 mark), provide independence (1 mark), and easy design and construction (1 mark).

Disadvantages: Performance penalty (1 mark), lack of efficiency (1 mark) and high overhead (1 mark).

 
(b) List the protocol stack for a web application and an E mail application, assuming an ethernet network. [2]
Web application: HTTP/TCP/IP/ETHERNET (1 mark).

E mail application: SMTP/TCP/IP/ETHERNET (1 mark).

Class A 0 Network: 7 bits Host: 24 bits

 
(c) With the aid of a suitable diagram, describe the three IP Address formats used for addressing single hosts. From these, identify which is the most commonly encountered. Also explain briefly the IP address format used for multiple destinations. [8]
 
Class B 10 Network: 14 bits Host: 16 bits
Class C 110 Network: 21 bits Host: 8 bits

(2 marks) each.

Class C, IP address format is the most commonly used (1 mark). Class D, IP address format is used for multicast addressing, (identified by 111 in the high-order bits) i.e., addressing with multiple destinations (1 mark).

 
(d) Describe the main properties of routers used in wide area networks, and explain briefly the concept of hierarchical routing used in the Internet. [6]
Allow up to (3 marks) for these, or other relevant points:

Provide compatibility and connection between networks using different protocols (1 mark). Finds suitable path from source to destination and moves packets through the network (1 mark). Insulates the transport client with details / topology of the underlying network (1 mark).

Allow up to (3 marks) for these, or other relevant points:

The Internet is a hierarchy of networks (1 mark). To reduce complexity, each router contains detailed information only on a restricted subset of the Internet (1 mark). If a router gets a packet destined for an unknown address, it forwards that packet to a default address (1 mark)—which will be a router at a higher level of the hierarchy, able to route the packet appropriately (1 mark).