3com 4210 PWR 9-Port 3CR17341-91-ME User Manual

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Domain name system (DNS) is a mechanism used for TCP/IP applications to provide 
domain name-to-IP address translation. With DNS, you can use memorizable and 
meaningful domain names in some applications and let the DNS server resolve it 
into correct IP addresses.

There are two types of DNS services, static and dynamic. Each time the DNS server 
receives a name query, it checks its static DNS database before looking up the 
dynamic DNS database. Reduction of the searching time in the dynamic DNS 
database would increase efficiency. Some frequently used addresses can be put in 
the static DNS database.

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The static domain name resolution means manually setting up mappings between 
domain names and IP addresses. IP addresses of the corresponding domain names 
can be found in the static domain name resolution table for applications, such as 
Telnet.

Dynamic domain name resolution is implemented by querying the DNS server. The 
resolution procedure is as follows:

1 A user program sends a name query to the resolver in the DNS client.

2 The DNS resolver looks up the local domain name cache for a match. If a match is 

found, it sends the corresponding IP address back. If not, it sends the query to the 
DNS server.

3 The DNS server looks up its DNS database for a match. If no match is found, it 

sends a query to a higher-level DNS server. This process continues until a result, 
success or failure, is returned.

4 The DNS client performs the next operation according to the result.