Islam OSAMA has sent an E-Mail contsining the following questions regarding Problem 2 in Sheet#3:

Q1: When deducing the IP address of the destination, why FOCEU DNS doesn't ask directly the root DNS instead of asking EUN DNS ?

Q2: Why the Internet Lab & the research Lab don't have their own local DNS ?

The following are the answers:

A1: DNS servers are organized in a hierarchical manner, in order to reduce storage requirements and to reduce traffic sent to upper layer DNS's. Thus, each DNS would contain the IP address mapping to the names under its domain. And if a certain name does not exist in the mapping table, it would check first the DNS directly above it. This would reduce the traffic that the Root Server would otherwise recive.

A2: The Internat Lab (as well as the Research Lab) normally have few entities with real IP addresses. In such case, it would be wasting too much money (DNS server cost) to place these few IP addresses and their mapped URL's in a machine. Rather, it makes more sense to gather several real IP's from Internet Lab and other similar locations and group them in ONE DNS server.