How is domain name resolved in a large network the Internet

1
Chapter 14

• How is domain name resolved in a large network
  (the Internet)?

2
DNS

• DNS server
• A computer running DNS server software to resolve
  domain name

3
DNS

• In a small network, one DNS server may be
  sufficient
• However, in a large network (e.g., the Internet),
  a number of DNS servers may be required
• A number of DNS servers form a network of DNS
  servers that work together to provide domain name
  resolution service for the large network.
• How do they work together to resolve a domain
  name?

4
DNS

• DNS servers in a network are structured in a
  hierarchy
• DNS server hierarchy (structure) follows domain
  name hierarchy (structure)

5
Domain Name Hierarchy (Structure)

• Domain name uses hierarchical name space
  (instead of flat name space)
• A domain name must consist of several parts
  (levels) organized into a tree-like structure
  (DNS tree)
• General structure
• Domain name A.B.C.D.E.F
• F Top level domain
• E Second level domain
• D Third level domain
• C Fourth level domain
• A Host name
• Example
• www.yahoo.com (com top level domain, yahoo
  second level domain, www host name)

6
Domain Name Hierarchy (Structure)

• Fully qualified domain name (FQDN)
• A complete domain name
• FQDN must start with host name and end with top
  level domain
• Examples
• A.B.C.D is a FQDN if A is host name and D is top
  level domain
• www.yahoo.com is FQDN if www is host name and com
  is top level domain
• yahoo.com is NOT FQDN because yahoo is second
  level domain and com is top level domain

7
Domain Name

• Domain name structure
• ROOT () (does not have physical presence)
• TOP LEVEL DOMAIN (examples of Internet top level
  domain com, edu, net, gov)
• SECOND LEVEL DOMAIN (examples of Internet second
  level domain microsoft, uah, cnn)
• THIRD LEVEL DOMAIN
• HOST NAME

8
Domain Name Structure
ROOT
Top Level Domain
Second Level Domain
Third Level Domain

Host Name
9
DNS Server Structure
ROOT DNS server (Internet 13 root servers)
Top Level Domain DNS server
Second Level Domain DNS server
Third Level Domain DNS server

Host Name
10
Domain Name Resolution How does a Computer
Resolve a Domain Name?

• The local DNS server has the addresses of the
  Root DNS server
• The ROOT DNS server has the addresses of the top
  level DNS server (e.g., .com, .net, .gov, .edu)
• The top level DNS server has the addresses of the
  second level DNS server (e.g., .com DNS server
  has the addresses of yahoo.com, cnn.com,
  amazon.com, google.com, etc.)
• The second level DNS server has the addresses of
  the third level DNS server and some hosts (e.g.,
  yahoo.com DNS server has the address of
  www.yahoo.com uah.edu DNS server has the
  addresses of www.uah.eu, webpages.uah.edu,
  cba.uah.edu)

11
Domain Name Resolution How does a Computer
Resolve a Domain Name?

• Case You open your Web browser and type
  www.yahoo.com. How does your computer resolve
  this domain name?
• Step 1. Your computer will contact the local DNS
  server available in the TCP/IP properties (see
  page 261, figure 11.27) (Actually, before
  contacting the local DNS server, your computer
  will try to use its HOSTS file)
• Step 2. If the DNS server does not have the
  www.yahoo.com, this DNS server will provide the
  address of the ROOT DNS server to your computer
• Step 3. Your computer will then contact the ROOT
  DNS server. The ROOT DNS server will provide the
  address of the .com DNS server to your computer

12
Domain Name Resolution How does a Computer
Resolve a Domain Name?

• Step 4. Your computer will then contact the .com
  DNS server. The .com DNS server will provide the
  address of the yahoo.com DNS server to your
  computer
• Step 5. Your computer will then contact the
  yahoo.com DNS server. The yahoo.com DNS server
  will provide the IP address of the www.yahoo.com
  (the yahoo.com DNS server will RESOLVE the
  www.yahoo.com) Web server to your computer
• Step 6. Your computer will then contact the
  www.yahoo.com Web server by using the IP address
  provided by the yahoo.com DNS server
• Step 7. The IP address of the www.yahoo.com will
  be stored in DNS resolver cache for future usage
• To see your computers DNS resolver cache open
  command prompt ? type ipconfig/displaydns

13
Domain Name Resolution How does a Computer
Resolve a Domain Name?
Try HOSTS File
Try Local DNS Server
Contact Root DNS Server
Contact Top Level DNS Server
Contact Second Level DNS Server

Contact Host Name Server
14
Root DNS Server
.com DNS Server
yahoo.com DNS Server
www.yahoo.com DNS Server
Local DNS Server