MCSA/MCSE Self-Paced Training Kit (Exam 70-270): Installing ...

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Lesson 3 Overview of Name Resolution
13-35
When a name server receives a query result, the following process takes place (see Figure
13-11):
1. The name server caches the query result for a specified amount of time, which is
referred to as Time to Live (TTL).
Note The zone that provided the query results specifies the TTL. The default value is 60
minutes.
2. After the name server caches the query result, TTL starts counting down from its
original value.
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
server
Query
Result
Cache
Result
TTL
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
Name
server
TTL
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.