TCP IP Illustrated

We can still access the host gemini using the other

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Unformatted text preview: nswer in Figure 14.10 and the first line of output from the host command are the IP address that shares the same subnet (140.252.1) as the requesting host. This is not an accident. If the name server and the host issuing the query are on the same network (or subnet), then BIND sorts the results so that addresses on common networks appear first. We can still access the host gemini using the other address, but it might be less efficient. Using traceroute in this instance shows that the normal route from subnet 140.252.1 to 140.252.3 is not through the host gemini, but through another router that's connected to both networks. So in this case if we accessed gemini through the other IP address (140.252.3.54) all the packets would require an additional hop. We return to this example and explore the reason for the alternative route in Section 25.9, when we can use SNMP to look at a router's routing table. There are other programs that provide easy interactive access to the DNS. nslookup is supplied with most implementations of the DNS. Chapter 10 of [Albitz and Liu 1992] provides a detailed description of how to use this program. The dig program ("Domain Internet Groper") is another publicly available tool that queries DNS servers, doc ("Domain Obscenity Control") is a shell script that uses dig and diagnoses misbehaving domains by sending queries to the appropriate DNS name servers, and performing simple analysis of the responses. See Appendix F for details on how to obtain these programs. The final detail to account for in this example is the size of the UDP data in the reply: 69 bytes. We need to know two points to account for these bytes. 1. The question is returned in the reply. 2. There can be many repetitions of domain names in a reply, so a compression scheme is used. Indeed, in our example, there are three occurrences of the domain name gemini.tuc.noao.edu. The compression scheme is simple. Anywhere the label portion of a domain name can occur, the single count byte (which is between 0 and 63) has its two high-order bits tu...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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