1.11: DNS

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IP addresses are hard to remember (nearly impossible in IPv6). The domain name system, or DNS (7.8 DNS), comes to the rescue by creating a way to convert hierarchical text names to IP addresses. Thus, for example, one can type www.luc.edu instead of 147.126.1.230. Virtually all Internet software uses the same basic library calls to convert DNS names to actual addresses.

One thing DNS makes possible is changing a website’s IP address while leaving the name alone. This allows moving a site to a new provider, for example, without requiring users to learn anything new. It is also possible to have several different DNS names resolve to the same IP address, and – through some modest trickery – have the http (web) server at that IP address handle the different DNS names as completely different websites.

DNS is hierarchical and distributed. In looking up cs.luc.edu four different DNS servers may be queried: for the so-called “DNS root zone”, for edu, for luc.edu and for cs.luc.edu. Searching a hierarchy can be cumbersome, so DNS search results are normally cached locally. If a name is not found in the cache, the lookup may take a couple seconds. The DNS hierarchy need have nothing to do with the IP-address hierarchy.