s naming convention naturally defines a hierarchy. Zones Domain Name Service (DNS) is a huge distributed database that contains information of each domain name. Each server maintains a part of the database called zone. Usually, a zone contains information of one domain. However, one zone may contain information about many (sub)domains. Each information element is stored in a record that contains at least a domain name and type and type-specific information. Delegation When a part of a zone is maintained separately, it is dele- gated to a new nameserver that will have authority of that part of domain name space. The original zone will have a nameserver record for the dele- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Function of GPRS Elements Function of GPRS Elements gated domain and the new subzone will have a new Source of Authority (SOA) record. Client DNS client is implemented as a resolver library. Application pro- grams use function calls like gethostbyname to find an IP address repre- senting a domain name. The name may be specified only partially and in that case, the resolver library appends a configured local domain name(s) at the end of the name. For example, the user may give the following command: ping hobbes The resolver library appends the domain search list and will queries the nameserver with the following: 'hobbes.sonera.com hobbes.sonera. hobbes' Domain names ending with a period are called fully qualified domain names. Search list components are not appended on these names. Server DNS server takes care of name service queries sent by clients. The query is answered by using either locally stored information or by asking the information from other nameservers. Sending queries to other name- servers is potentially time and network resources consuming task. Storing previously queried information in a local cache optimises 