64 Networking and Online Games: Understanding and Engineering
66 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games 4.3.4 Domain Name System As noted earlier in this chapter, IP addresses are not the same as FQDNs (often referred to simply as domain names). Domain names are human-readable, text-form names that indirectly represent IP addresses. The DNS is a distributed, automated, hierarchical look-up and address mapping service [RFC1591]. People typically use domain names to inform an application of a remote Internet destination, and the applications then use the DNS to perform on-demand mappings of domain names to IP addresses. This level of indirection allows consistent use of well-known domain names to identify hosts, while allowing a host s IP address to change over time (for whatever reason). Two forms of hierarchy exist in the DNS hierarchy in the structure of names themselves and a matching hierarchy in the distributed look-up mechanism. 4.3.4.1 Domain Name Hierarchy Domain names are minimally of the form name . tld. where tld. specifies one of a handful of Top Level Domains (TLDs) and name. is an identifier registered under the specified top-level domain. Examples of generic three letter TLDs (gTLDs) include com, edu, net, org, int, gov, and mil. Country code TLDs (ccTLDs) are constructed from standard ISO-3166 two letter country codes (e.g. au, uk, fr, and so on) [ISO3166]. The nested hierarchy is read from right to left, and name. mayitself bebrokenupinto multiple levels of subdomains. Some TLDs are relatively flat (for example, the com TLD), with companies and originations around the world able to register second-level domains immediately under com . Country code TLDs have varied underlying structures, sometimes replicating a few of the existing three letter TLDs as second-level domains (for example, Australia registers domain names under a range of second-level domains including com.au , edu.au , and so on.) The hierarchical structure reflects the administrative hierarchy of authority associated with assigning names to IP addresses. For example, consider an address like mail.accounting.bigcorp.com . The managers for com have delegated all naming under bigcorp.com to a second party (most likely the owners of BigCorp, Inc. ). BigCorp no doubt has various internal departments, including the Accounting department. Someone in the accounting department has been delegated authority for naming under accounting.bigcorp.com , and they have assigned a name for the mail server in the accounting department. Figure 4.18 represents the relationships between the subdomains discussed so far. A domain name hierarchy is independent of the hierarchy of IP addresses and sub- nets discussed earlier in this chapter. For example, onemachine.bigcorp.com might well be on an entirely different IP subnet (indeed, even a different country) from othermachine.bigcorp.com. Domain name registration has become a commercial business in its own right, and multiple registrars jointly manage different sections of the DNS. Up-to-date information on registrars and domain assignment policies can be found in the Internet Assigned Numbers Authority web site, http://www.iana.org.
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