DNS (Domain Name System)

• Created to work generally with the application layer to resolve host names

IP

• IPv4: 4 octets/bytes, usually written in decimal
  • Not enough for each person in the world to have a unique IP address (more on NAT later)
• IPv6: 16 bytes, usually written in hex
  • ~340 undecillion addresses
• IP is part of the network layer (an IP header is added, creating a datagram)

Routing requests

Application Layer

• Establishes HOSTNAME, Port, initializes socket connection
• Calls DNS, sends IP to transport layer
  Transport Layer
• Adds header to ADU, sends to segment to Network Layer (with IP address, Port)
  Network Layer
• Adds header to segment, containing IP address
• Sends IP datagram to DDL Layer

DNS

• Distributed databases
  • 13 root servers (routers choose closest)
    • Thousands of actual locations
    • One IP associated with many locations per root server
  • Hierarchy extends into TLD DNS servers, which root servers query
  • Local name server
    • Each ISP has one
• Application-layer
• No user interaction
• Resolution types
  • Iterative: "tell who knows"
  • Recursive: "give the final answer" (used by end users)

Record types

• A (AAAA for IPv6)
  • name: hostname
  • value: IP address
• NS
  • name: domain
  • value: hostname of authoritative name server for this domain
• CNAME
  • Alias name for the real name
  • value: real name
• MX
  • value: name of mailserver
• SOA
  • value: info regarding authoritative name server