Dec1999 AN215 : Advanced Networking (Question 5)Suggested Solutions

Local area networks can be connected together using devices such as bridges,routers,or gateways.The choice of which form of connection to use will depend on the nature of the networks involved.
(a)	For each of the three devices mentioned above,state at which level(s)of the OSI model it operates,giving reasons for your answers. Bridge operates at the data-link layer (1 mark).It connects together networks without the intelligence needed for the network layer (1 mark). Router operates at the network layer (1 mark).It provides networking among dissimilar networks,to the extent of being able to divide up packets and resolving different addressing schemes (1 mark). Gateway operates at all seven layers (1 mark).It connects proprietary networks by translating between different protocols as necessary (1 mark). Award marks for these or other valid explanations.	[6]
(b)	A bridge may be used to divide a LAN into several interconnected LANs.Describe three reasons for using a bridge in this way. Award marks for points from the following,or other relevant points. Improved performance (1 mark)—when most network traffic on each subnetwork is local to that subnet (1 mark). Robustness (1 mark)—if a single subnetwork fails,only that network is down,and the rest of the LAN is unaffected (1 mark). Security (1 mark)—to prevent sensitive information being placed on a general-purpose LAN (1 mark).	[6]
(c)	For each of the following situations,state which means of connection would be the most appropriate. (i)The connection between a training company ’s backbone ethernet network and the sub-network (a segment)used in a particular computer laboratory.[1 mark ] sub-network (a segment)used in a particular computer laboratory.[1 mark ] Bridge (1 mark) (ii)The connection between a company ’s private network and the Internet. [1 mark ] Gateway (1 mark)[potentially router +firewall (1 mark)] (iii)The connection between the company ’s network and a special proprietary network used in the headquarters,which is incompatible.[1 mark ] Gateway (1 mark)	[3]