TCP IP Illustrated

Section 145 associates the name with the ip address

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Unformatted text preview: s name. This is because the reverse name lookup (return the name, given the IP address. Section 14.5), associates the name with the IP address, but the forward lookup (given the name, return the IP address) fails. The forward mapping and reverse mapping are two separate files in the DNS (Domain Name System) and not all administrators keep the two synchronized with each other. It's not uncommon to have one direction work and the other direction fail. Something that we haven't seen before is the asterisk (*) printed for the first RTT for the TTL of 8. This indicates that a timeout occurred and no response was received within 5 file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (12 of 17) [12/09/2001 14.46.48] Traceroute Program seconds for this probe. Another point that we can infer from a comparison of this figure and Figure 8.4 is that the router nsn-FIX-pe.sura.net is connected to both the NSFNET and the NASA Science Internet. traceroute Examples with Strict Source Routing The -G option in the author's version of traceroute is identical to the -g option described earlier, but the source route is strict instead of loose. We can use this to see what happens when an invalid strict source route is specified. Recall from Figure 8.5 that the normal sequence of routers for datagrams from the author's subnet to the NSFNET is through netb, gateway, butch, and gabby. (We've omitted the domain suffixes, .tuc.noao.edu and .telcom.arizona.edu, in all the output below to make it easier to read.) We specify a strict source route that omits butch, trying to go directly from gateway to gabby. We expect this to fail, as shown in Figure 8.9. sun % traceroute -G netb -G gateway -G gabby westgate traceroute to westgate (192.80.43.2), 30 hops max. 40 byte packets 1 netb (140.252.1.183) 272 ms 257 ms 261 ms 2 gateway (140.252.1.4) 263 ms 259 ms 234 ms 3 gateway (140.252.1.4) 263 ms !S * 235 ms !S Figure 8.9 traceroute with a strict source rou...
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