Chapter 25 discusses the Domain Name System (DNS). DNS is a client/server application that provides name services for other applications. It enables the use of applicationlayer addresses, such as an email address, instead of network layer logical addresses. | Chapter 25 Domain Name System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 25. Figure Example of using the DNS service 25. 25-1 NAME SPACE To be unambiguous, the names assigned to machines must be carefully selected from a name space with complete control over the binding between the names and IP addresses. Flat Name Space Hierarchical Name Space Topics discussed in this section: 25. 25-2 DOMAIN NAME SPACE To have a hierarchical name space, a domain name space was designed. In this design the names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127. Label Domain Name Domain Topics discussed in this section: 25. Figure Domain name space 25. Figure Domain names and labels 25. Figure FQDN and PQDN 25. Figure Domains 25. 25-3 DISTRIBUTION OF NAME SPACE The information contained in the domain name space must be stored. However, it is very inefficient and also unreliable to have just one computer store such a huge amount of information. In this section, we discuss the distribution of the domain name space. Hierarchy of Name Servers Zone Root Server Primary and Secondary Servers Topics discussed in this section: 25. Figure Hierarchy of name servers 25. Figure Zones and domains 25. A primary server loads all information from the disk file; the secondary server loads all information from the primary server. When the secondary downloads information from the primary, it is called zone transfer. Note 25. 25-4 DNS IN THE INTERNET DNS is a protocol that can be used in different platforms. In the Internet, the domain name space (tree) is divided into three different sections: generic domains, country domains, and the inverse domain. Generic Domains Country Domains Inverse Domain Topics discussed in this section: 25. Figure DNS IN THE INTERNET 25. Figure | Chapter 25 Domain Name System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 25. Figure Example of using the DNS service 25. 25-1 NAME SPACE To be unambiguous, the names assigned to machines must be carefully selected from a name space with complete control over the binding between the names and IP addresses. Flat Name Space Hierarchical Name Space Topics discussed in this section: 25. 25-2 DOMAIN NAME SPACE To have a hierarchical name space, a domain name space was designed. In this design the names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127. Label Domain Name Domain Topics discussed in this section: 25. Figure Domain name space 25. Figure Domain names and labels 25. Figure FQDN and PQDN 25. Figure Domains 25. 25-3 DISTRIBUTION OF NAME SPACE The information contained in the domain name space must
