[align=center]DNS[/align]
[align=left]DNS name structure
In the early days of the Internet, all host names and their associated IP addresses were recorded in a single file called hosts.txt, maintained by the Network Information Centre in the USA. Not surprisingly, as the Internet grew so did this file, and by the mid-80's it had become impractically large to distribute to all systems over the network, and impossible to keep up to date. The Internet Domain Name System (DNS) was developed as a distributed database to solve this problem. It's primary goal is to allow the allocation of host names to be distributed amongst multiple naming authorities, rather than centralised at a single point.
DNS names are constructed hierarchichally. The highest level of the hierarchy being the last component or label of the DNS address. Labels can be up to 63 characters long and are case insensitive. A maximum length of 255 characters is allowed. Labels must start with a letter and can only consist of letters, digits and hyphens. [Unfortunately some administrators construct names that start with digits. This is wrong and can easily cause problems with software that simply inspects the first character of a host address to determine whether a DNS name or an IP address has been quoted.]
Note In the early days of the Internet users in at least one country (the United Kingdom) adopted a similar scheme with the highest hierarchical level appearing first rather than last. I.e. uk.ac.wlv.scit.sun rather than sunc.scit.wlv.ac.uk. This practice is, fortunately, obsolete.
DNS addresses can be relative or fully qualified. A fully qualified address includes all the labels and is globally unique. A relative address can be converted by appending the local domain information. For example sunc.scit.wlv.ac.uk is a fully qualified name for the host sunc in the domain scit.wlv.ac.uk. Strictly there should be a stop at the end of a fully qualified name but this is often overlooked.
The final most significant label of a fully qualified name can fall into one of three classes
arpa
This is a special facility used for reverse translation, i.e. going from IP address to fully qualified domain address. If everything is properly configured a suitably framed query for 1.4.220.134.in-addr.arpa will return sunc.scit.wlv.ac.uk. Details of this will be described later.
3 letter codes
The DNS was orginally introduced in the United States of America and the final component of an address was intended to indicate the type of organisation hosting the computer. Some of the three letter final labels (edu, gov, mil) are still only used by organisations based in the USA, others can be used anywhere in the world.
The three letter codes are
code meaning
com Commercial. Now international.
edu Educational.
gov Government.
int International Organisiation.
mil Military.
net Network related.
org Miscellaneous Organisation.
Two letter codes
The final two letter codes indicate the country of origin and are defined in ISO 3166 with the minor exception that uk is used for the United Kingdom rather than gb although there are some .gb sites. [This apparently happened because the ISO committee was unaware that Northern Ireland was part of the United Kingdom but not part of Great Britain.]
The two letter code us is used by some sites in the United States of America.
In some countries there are sub-domains indicating the type of organisation such as ac.uk, co.uk, sch.uk in the United Kingdom and edu.au and com.au in Australia. Most European countries have not adopted this useful practice. A fuller discussion of the United Kingdom DNS domains is provided by To obtain a domain address it is necessary to identify the administrator of the required domain and then all that is basically necessary is to send the administrator the required code and the associated IP address and they will, if they accept the request, include the details in their databases. Conditions for acceptance vary widely between administrators, the administrators for the com and org being, apparently, quite happy to accept anything from anywhere.
A DNS server is just a computer that's running DNS software. Since most servers are Unix machines, the most popular program is BIND (Berkeley Internet Name Domain), but you can find software for the Mac and the PC as well.
DNS software is generally made up of two elements: the actual name server, and something called a resolver. The name server responds to browser requests by supplying name-to-address conversions. When it doesn't know the answer, the resolver will ask another name server for the information.
To see how it works, let's go back to the domain-name-space inverted tree.
When you type in a URL, your browser sends a request to the closest name server. If that server has ever fielded a request for the same host name (within a time period set by the administrator to prevent passing old information), it will locate the information in its cache and reply.
If the name server is unfamiliar with the domain name, the resolver will attempt to "solve" the problem by asking a server farther up the tree. If that doesn't work, the second server will ask yet another - until it finds one that knows. (When a server can supply an answer without asking another, it's known as an authoritative server.)
Once the information is located, it's passed back to your browser, and you're sent on your merry way. Usually this process occurs quickly, but occasionally it can take an excruciatingly long time (like 15 seconds). In the worst cases, you'll get a dialog box that says the domain name doesn't exist - even though you know damn well it does.
This happens because the authoritative server is slow replying to the first, and your computer gets tired of waiting so it times-out (drops the connection). But if you try again, there's a good chance it will work, because the authoritative server has had enough time to reply, and your name server has stored the information in its cache.
DNS Structure
The DNS is arranged as a hierarchy, both from the perspective of the structure of the names maintained within the DNS, and in terms of the delegation of naming authorities. At the top of the hierarchy is the root domain "." which is administered by the Internet Assigned Numbers Authority (IANA). Administration of the root domain gives the IANA the authority to allocate domains beneath the root. The process of assigning a domain to an organisational entity is called delegating, and involves the administrator of a domain creating a sub-domain and assigning the authority for allocating sub-domains of the new domain the subdomain's administrative entity.
This is a hierarchical delegation, which commences at the "root" of the Domain Name Space ("."). A fully qualified domain name, is obtained by writing the simple names obtained by tracing the DNS hierarchy from the leaf nodes to the root, from left to right, separating each name with a stop ".", eg. fred.xxxx.edu.au. is the name of a host system (huxley) within the XXXX University (xxx), an educational (edu) institution within Australia (au).
The sub-domains of the root are known as the top-level domains, and include the edu (educational), gov (government), and com (commercial) domains. Although an organisation anywhere in the world can register beneath these three-character top level domains, the vast majority that have are located within, or have parent companies based in, the United States. The top-level domains represented by the ISO two-character country codes are used in most other countries, thus organisations in Australia are registered beneath au.
The majority of country domains are sub-divided into organisational-type sub-domains. In some countries two character sub-domains are created (eg. ac.nz for New Zealand academic organisations), and in others three character sub-domains are used (eg. com.au for Australian commercial organisations). Regardless of the standard adopted each domain may be delegated to a separate authority.
Organisations that wish to register a domain name, even if they do not plan to establish an Internet connection in the immediate short term, should contact the administrator of the domain which most closely describes their activities.
Even though the DNS supports many levels of sub-domains, delegations should only be made where there is a requirement for an organisation or organisational sub-division to manage their own name space. Any sub-domain administrator must also demonstrate they have the technical competence to operate a domain name server (described below), or arrange for another organisation to do so on their behalf.
Domain Name Servers
The DNS is implemented as collection of inter-communicating nameservers. At any given level of the DNS hierarchy, a nameserver for a domain has knowledge of all the immediate sub-domains of that domain.
For each domain there is a primary nameserver, which contains authoritative information regarding Internet entities within that domain. In addition Secondary nameservers can be configured, which periodically download authoritative data from the primary server. Secondary nameservers provide backup to the primary nameserver when it is not operational, and further improve the overall performance of the DNS, since the nameservers of a domain that respond to queries most quickly are used in preference to any others. Thus, in addition to having a primary nameserver on site, each organisation should have at least one secondary on site, and another elsewhere on the Internet, preferably well connected. This is particularly important for entities with slow speed or dial-up Internet connections to reduce use of their link to support the DNS. [/align]
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