The Domain Name Service (DNS)

The Domain Name Service (DNS) is a hierarchical naming system replacing the older Hosts name list, which ceased to be useful when host numbers outgrew useful unique host names. The domain name consists of a hostname, combined with a registered domain name and a Top-Level Domain (TLD). For example cordon1.freeserve.co.uk consists of the host name, cordon1, the registered domain name, freeserve and the Top-Level Domains co and uk. The registered domain must be unique, hence the need for registration with a centralised authority.

There are too many registered domains for a single DNS server; therefore servers have a similar hierarchy to the name structure.

Root level DNS servers are each identified by a letter of the alphabet. These servers contain the IP addresses of the suffix (Top-Level Domain) servers. These servers in turn hold the

IP addresses for the DNS servers of each registered domain. These local DNS servers contain the IP addresses of web, mail, FTP, Gopher servers within that organisation and depending on the size of the domain they may point to other local DNS servers within the domain.

Many domains have a secondary DNS server with a copy of the primary DNS serverís zone files, which comes into play when the primary server fails. Zone files are transferred between primary and secondary servers using zone transfers.

Caching-only severs are not registered with the next higher DNS server in the hierarchy, they are preconfigured with the IP addresses of nine root-level DNS servers so that they can find IP addresses when necessary. The addresses found in this way are stored in its cache for future reference

When a web browser contacts a local DNS server for an IP address corresponding to a Fully Qualified Domain Name (FQDN), it first checks its own zone files. If no match is found the local DNS server asks the suffix (TLD) server responsible for the TLD in the FQDN. This server does not have a record for the FQDN but it does have the IP address of the DNS server responsible for the registered domain name found in the FQDN and it provides this to the local DNS server. This server now contacts the registered domain DNS server asking for the IP address corresponding to the FQDN, that server provides the IP address. The local DNS server provides this address to the web browser, which now can retrieve the required web document.