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Lesson 2: Understanding Name Resolution
Name resolution is the process of resolving names to IP addresses. Name resolution is similar to looking up a name in a telephone book, where the name is associated with a telephone number. For example, when you connect to the Microsoft Web site, you use the name www.microsoft.com. DNS resolves www.microsoft.com to its associated IP address. The mapping of names to IP addresses is stored in the DNS distributed database.
DNS name servers resolve forward and reverse lookup queries. A forward lookup query resolves a name to an IP address. A reverse lookup query resolves an IP address to a name. A name server can resolve a query only for a zone for which it has authority. If a name server can't resolve the query, it passes the query to other name servers that can resolve the query. The name server caches the query results to reduce the DNS traffic on the network.
After this lesson, you will be able to
- Explain the name resolution process.
Estimated lesson time: 5 minutes
The DNS Service uses a client/server model for name resolution. To resolve a forward lookup query, which resolves a name to an IP address, a client passes a query to a local name server. The local name server either resolves the query or queries another name server for resolution.
Figure 8.3 represents a client querying the name server for an IP address of www.microsoft.com. The numbers in the figure depict the following activities:
Figure 8.3 Resolving a forward lookup query
When a name server is processing a query, it might be required to send out several queries to find the answer. With each query, the name server discovers other name servers that have authority for a portion of the domain name space. The name server caches these query results to reduce network traffic.
When a name server receives a query result, the following actions take place (see Figure 8.4):
NOTE
The zone that provided the query results specifies the TTL. The default value is 60 minutes.
Caching query results enables the name server to resolve other queries to the same portion of the domain name space quickly.
Figure 8.4 Caching query results
NOTE
Shorter TTL values help ensure that data about the domain name space is more current across the network. Shorter TTL values do increase the load on name servers, however. A longer TTL value decreases the time required to resolve information. However, if a change does occur, the client will not receive the updated information until the TTL expires and a new query to that portion of the domain name space is resolved.
A reverse lookup query maps an IP address to a name. Troubleshooting tools, such as the nslookup command-line utility, use reverse lookup queries to report back host names. Additionally, certain applications implement security based on the ability 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, a special second-level domain called in-addr.arpa was created.
The in-addr.arpa domain follows the same hierarchical naming scheme as the rest of the domain name space; however, it is based on IP addresses, not domain names:
For example, Figure 8.5 shows a dotted-decimal representation of the IP address 169.254.16.200. A company that has an assigned IP address range of 169.254.16.0 to 169.254.16.255 with a subnet mask of 255.255.255.0 will have authority over the 16.254.169.in-addr.arpa domain.
Figure 8.5 The in-addr.arpa domain
In this lesson, you learned that name resolution is the process of resolving names to IP addresses and that the mapping of names to IP addresses is stored in the DNS distributed database. You learned that DNS name servers resolve forward lookup queries and what happens when a client queries the name server for an IP address. You also learned about name server caching and that the name server caches the query results to reduce the DNS traffic on the network.
You also learned that in addition to forward lookup queries, DNS name servers resolve reverse lookup queries. A reverse lookup query resolves an IP address to a name. Because the DNS distributed database is indexed by name and not by IP address, a special second-level domain called in-addr.arpa was created. The in-addr.arpa domain follows the same hierarchical naming scheme as the rest of the domain name space; however, it is based on IP addresses instead of domain names.