DNS

Components of URL

http     :// homepages.gac.edu / ~sskulrat/index.html
|            |                      |
protocol     host                   file

can also specify port by adding :port at the end.

DNS, continued

In the beginning, resolving name to address was done centrally. The Network Information Center (NIC) had a file named HOSTS.TXT containing all hosts in the internet. Each host ftp’ed this file and saved a copy. This method does not scale well as network bandwidth grows as \(n^2\).
So DNS was developed as a distributed system with extensive caching. This can work because
- the IP-to-name mapping is more often queried than changed
- inconsistency is tolerable
DNS uses the service of UDP transport protocol, port 53

A tree showing example domain & subdomain names

                              .

        edu           com    org    us    fr   ch

gustavus      mit

           math   cs

Three types of name servers
- root DNS servers
- top-level domain (TLD) DNS servers
- authoritative DNS servers
Divided into hierarchical zones (root, edu, com, org, umn, mit, google)
Each zone separately administered.
Each zone served from several replicated servers.
Root zone: managed by 13 different organizations (a, b, c, …, m), highly replicated (over 400 root name servers in the world)