chap6

• Domain names


  Used to represent IP addresses

  
• Domain Name System (DNS)

  The service to resolve domain name to IP address

  
• Domain Names


  • www.yahoo.com
  • www.cnn.com
  • www.sp.edu.sg

  
• How DNS service works

  1. Domain name query sent

  2. DNS Server returns IP address

  3. Web browser requests web-page

  
• DNS service is responsible for domain name to address resolution

  
• DNS service


  • Uses servers to resolve domain names
  • Is a client/server service

  
• The DNS client runs as a service in the computer where the web browser is running
• Besides web-browsing, the DNS client also supports name resolution for other network applications and services
• DNS servers manage a massive database that maps domain names to IP addresses

  
• DNS servers store different types of resource records used to resolve names

  
• Resource Record Types

  • A (Address)
  • MX (Mail Exchange)
  • NS (Authoritative Name Server)
  • CNAME (Canonical Name)
• A (Address)

  An end device address
• MX (Mail Exchange)

  Maps a domain name to a list of mail exchange servers for that domain
• NS (Authoritative Name Server)

  A name server that can give an authoritative answer to a DNS query
• CNAME (Canonical Name)


  Also known as “Fully Qualified Domain Name”

  
• Domain names are strings of characters separated by dots

  
• Top-level Domain (TLD)


  The last word in a domain name
• Common TLDs

  • COM
  • NET
  • ORG
  • EDU
  • GOV
  • SG
  • MY
  • UK
• Each level in a domain name refers to a server or a group of servers that manage that domain level
• Host name


  The left-most word in the domain name
• A given domain can have many host names as long as they're all unique to that domain
• Windows Operating System has a utility called nslookup that can be used to manually query the name servers to resolve a domain name
• DNS servers can be internal or external
• Internal DNS server knows names and addresses of all internal hosts inside Company A’s network
• External DNS servers know names and addresses of Company A’s mail server, web server, and firewall
• When a computer connects to a network, the DHCP server sends network configuration information to the computer
• The configuration information includes one or more DNS servers that the computer should use for translation of DNS names to IP address
• DNS uses a hierarchical system to create a name database to support name resolution

  
• The Root DNS servers maintain records about how to reach the top-level domain (TLD) servers
• TLD servers have records about how to reach the secondary level domain servers
• When a DNS client makes a query, the DNS server looks at its own records first to see whether it can resolve the domain name
• If a match is found, the server stores the returned address in its cache
• Caching reduces both the DNS query traffic and the workloads of servers higher up the hierarchy
• All domain names need to be unique
• ICANN enforces uniqueness of domain names across the Internet

  
• Each domain registration becomes part of a central domain registration database known as the whois database
• Singapore Network Information Centre (SGNIC) Pte Ltd administers the Internet domain name space in Singapore
• SG Domain extensions

  • sg
  • com.sg
  • org.sg
  • edu.sg
  • gov.sg
  • net.sg
  • per.sg
  • 新加坡
  • ஼ங்கப்ௗர்
• Registration of domain name is done through registrars