Aug 1997 AN215: Advanced Networking (Question 2) Suggested Solutions

2.	Besides simple cable connections, two local area networks (LANs) may be connected by a bridge, a router or a gateway. The choice of which form of connection to use will depend on the nature of the two networks involved.

	(a) For each of these four, state at which level(s) in the OSI model it operates, giving reasons for your answers.		[8]
	Simple cable: physical only. No active components
	[2 marks]
	Bridge: operates at the data link layer (and involves physical layer connections too). Connects networks together, interpreting data, but without the intelligence needed for the network layer (e.g. no flow control)
	[2 marks]
	Router: operates at network layer. (and hence involves physical and data link abilities). Provides internetworking among dissimilar networks, to the extent of being able to divide up packets as necessary, and resolving different addressing schemes
	[2 marks]
	Gateway: operates at all seven levels. connect arbitrary different networks by translatig between different protocols as necessary.
	[2 marks]

	(b) A bridge may be used to subdivide a LAN into several interconnected LANs. Explain three reasons for using a bridge in this way.		[6]
		to improve performance on the subnetworks--when most of the traffic on each subnetwork is local to that subnet.
		[2 marks]
		to improve robustness, so that a single LAN component failure need not cause the whole network to fail.
		[2 marks]
		for secrurity; to prevent sensitive information being placed on a general-purpose LAN
		[2 marks]

	(c) For each of the following situations, state which means of connection would be most appropriate, and why.
	*The following answers are preferred; other well-argued* answers are acceptable if they demonstrate understanding of the technology.**
		(i) The connection between a training organisation's main (backbone) ethernet network, and the local network used in a particular computer laboratory/classroom.	[2]
		router: to provide connection, but offer protection against flooding of the backbone by spurious messages from those experimenting/hacking in the lab.
		[2 marks]
		(ii) The connection between a company's private network and the Internet.	[2]
		gateway: to permit careful control of which connections are allowed/denied into and out of the organisation
		[2 marks]
		(iii) The connection between the same backbone and a special proprietary network used in the headquarters' accounts department.	[2]
		router: technically suitable for providing access outwards from accouts department, and limit unwelcome access to the computers in the accounts department.
		[2 marks]