All routing across the network is done by means of the IP address associated with a packet. Since humans find it difficult to remember addresses like 126.96.36.199, a symbolic name register was set up at the NIC where people would say "I would like my host to be named 'uiucuxc'". Machines connected to the Internet across the nation would connect to the NIC in the middle of the night, check modification dates on the hosts file, and if modified move it to their local machine. With the advent of workstations and micros, changes to the host file would have to be made nightly. It would also be very labor intensive and consume a lot of network bandwidth. RFC-882 and a number of others describe domain name service, a distributed data base system for mapping names into addresses.
We must look a little more closely into what's in a name. First, note that an address specifies a particular connection on a specific network. If the machine moves, the address changes. Second, a machine can have one or more names and one or more network addresses (connections) to different networks. Names point to a something which does useful work (i.e. the machine) and IP addresses point to an interface on that provider. A name is a purely symbolic representation of a list of addresses on the network. If a machine moves to a different network, the addresses will change but the name could remain the same.
Domain names are tree structured names with the root of the tree at the right. For example:
is a machine called 'uxc' (purely arbitrary), within the subdomains method of allocation of the U of I) and 'uiuc' (the University of Illinois at Urbana), registered with 'edu' (the set of educational institutions).
A simplified model of how a name is resolved is that on the user's machine there is a resolver. The resolver knows how to contact across the network a root name server. Root servers are the base of the tree structured data retrieval system. They know who is responsible for handling first level domains (e.g. 'edu'). What root servers to use is an installation parameter. From the root server the resolver finds out who provides 'edu' service. It contacts the 'edu' name server which supplies it with a list of addresses of servers for the subdomains (like 'uiuc'). This action is repeated with the subdomain servers until the final subdomain returns a list of addresses of interfaces on the host in question. The user's machine then has its choice of which of these addresses to use for communication.
A group may apply for its own domain name (like 'uiuc' above). This is done in a manner similar to the IP address allocation. The only requirements are that the requestor have two machines reachable from the Internet, which will act as name servers for that domain. Those servers could also act as servers for subdomains or other servers could be designated as such. Note that the servers need not be located in any particular place, as long as they are reachable for name resolution. (U of I could ask Michigan State to act on its behalf and that would be fine). The biggest problem is that someone must do maintenance on the database. If the machine is not convenient, that might not be done in a timely fashion. The other thing to note is that once the domain is allocated to an administrative entity, that entity can freely allocate subdomains using what ever manner it sees fit.
The Berkeley Internet Name Domain (BIND) Server implements the Internet name server for UNIX systems. The name server is a distributed data base system that allows clients to name resources and to share that information with other network hosts. BIND is integrated with 4.3BSD and is used to lookup and store host names, addresses, mail agents, host information, and more. It replaces the /etc/hosts file for host name lookup. BIND is still an evolving program. To keep up with reports on operational problems, future design decisions, etc, join the BIND mailing list by sending a request to bind-request@ucbarpa.Berkeley.edu. It can also be obtained via anonymous FTP from ucbarpa.berkley.edu.
There are several advantages in using BIND. One of the most important is that it frees a host from relying on /etc/hosts being up to date and complete. Within the .uiuc.edu domain, only a few hosts are included in the host table distributed by SRI. The remainder are listed locally within the BIND tables on uxc.cso.uiuc.edu (the server machine for most of the .uiuc.edu domain). All are equally reachable from any other Internet host running BIND.
BIND can also provide mail forwarding information for interior hosts not directly reachable from the Internet. These hosts can either be on non-advertised networks, or not connected to a network at all, as in the case of UUCP-reachable hosts. More information on BIND is available in the "Name Server Operations Guide for BIND" in UNIX System Manager's Manual, 4.3BSD release.
There are a few special domains on the network, like SRINIC.ARPA. The 'arpa' domain is historical, referring to hosts registered in the old hosts database at the NIC. There are others of the form NNSC.NSF.NET. These special domains are used sparingly and require ample justification. They refer to servers under the administrative control of the network rather than any single organization. This allows for the actual server to be moved around the net while the user interface to that machine remains constant. That is, should BBN relinquish control of the NNSC, the new provider would be pointed to by that name.
In actuality, the domain system is a much more general and complex system than has been described. Resolvers and some servers cache information to allow steps in the resolution to be skipped. Information provided by the servers can be arbitrary, not merely IP addresses. This allows the system to be used both by non-IP networks and for mail, where it may be necessary to give information on intermediate mail bridges.