OBTAIN AN IP ADDRESS BY USING DHCPBEFORE YOU BEGIN THIS EXERCISE, YOU...

3. When TTL expires, the name server deletes the query result from its cache.

Caching query results enables the name server to resolve other queries to the same

portion of the domain namespace quickly.

Name serverQuery TTLResultResult TTLCache

F13us12

Figure 13-11 Names servers can cache query results for quicker subsequent access.Note Shorter TTL values ensure that data about the domain namespace is more current across the network. However, shorter TTL values cause the cached values to expire sooner and increase the DNS traffic. A longer TTL value causes the cached values to be retained longer, which decreases the DNS traffic but increases the risk of the entries becoming stale. If a change does occur, the client does not receive the updated information until the TTL expires and a new query to that portion of the domain namespace is resolved.

How a Reverse Lookup Query Works

A reverse lookup query maps an IP address to a name. Troubleshooting tools, such as

the nslookup command-line tool, use reverse lookup queries to report back host

names. Additionally, certain applications implement security based on the capability to

connect to names, not IP addresses.

Because the DNS distributed database is indexed by name and not by IP address, a

reverse lookup query would require an exhaustive search of every domain name. To

solve this problem, in-addr.arpa was created. This special second-level domain follows

the same hierarchical naming scheme as the rest of the domain namespace; however,

it is based on IP addresses, not domain names, as follows:

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Subdomains are named after the numbers in the dotted-decimal representation of

IP addresses.

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The order of the IP address octets is reversed.

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Companies administer subdomains of the in-addr.arpa domain based on their

assigned IP addresses and subnet mask.

For example, Figure 13-12 shows a dotted-decimal representation of the IP address