Overview

• Homework
• Internet
  • Architecture
  • Domain Names
  • IP Addresses

Homework

• Due Monday, Oct 26
• 5-1
• 5-7
• read "man nslookup" and use nslookup to get the IP addresses for:
  • uhics.ics.hawaii.edu
  • prep.ai.mit.edu
  • the DNS server your "nslookup" is using.
  turn in the 3 IP addresses, and state for each what class the IP address belongs to.

Internet Architecture: Protocols

• FTP, Telnet, HTTP, SMTP, NFS, ...
• TCP, UDP
• IP
• Link layer

Internet Architecture: Networks

• Collection of Autonomous Systems (ASs)
• Each AS is a collection of networks
• Routing:
  • To destination AS
  • To destination network
  • To destination host

Internet Domain Names

• example, uhics.ics.hawaii.edu
• strictly hierarchical naming system:
  • top level has .edu, .com, .gov, .mil, .org, .int, .net, and all countries
  • each level has its own subdivisions, e.g. .edu has hawaii.edu, cmu.edu, unc.edu, whereas .gov has whitehouse.gov, senate.gov, hawaii.gov
• each domain name belongs to an organization, e.g. hawaii.edu belongs to the University of Hawaii, ics.hawaii.edu belongs to the ICS department

Domain Name Service (DNS)

• convert domain names (human readable) to IP addresses (uniquely identifying host)
• DNS is an implementation of a hierarchical, replicated, distributed database
• database is read-only for DNS
• at least two servers for each domain
• each host knows how to reach "its" DNS server(s)
• each DNS server is either at the root of the tree, or knows how to reach a DNS server higher up the tree
• each DNS server also knows what DNS servers are directly below it in the tree

DNS resolution

• looking for translation of www.inria.fr:
• I check my cache
• I ask my DNS server for the translation
• my DNS server either:
  • asks its server for the translation, or
  • gives me the name of its server
• eventually, the request reaches one of the root servers for .fr
• the root server for .fr transfers the request to inria.fr
• inria.fr provides the reply, either directly to me or through the servers

DNS questions

• distributed?
• central point of failure?
• stateless?

IP Addresses

• 32 bits, eg 128.171.44.7
• each packet must be self-routing based only on the address
• impossible for routers to route to 2^{32} individual hosts (especially in 1969)
• also impossible to exchange information about 2^{32} individual hosts
• so, give the IP address structure:
  • a network part
  • a host part
• from outside, only route to networks
• within a network, route to host
• two-level hierarchy

IP Host Routing

• each host is configured with the IP address of a default router, which must be on the same network
• to send packet to a destination host,
• compare destination address with your destination address:
  • if network part is the same, use network-specific mechanism (e.g. for ethernet, use ARP) to send packet directly to host
  • otherwise, use network-specific mechanism to send packet to router
• so, IP host routing is always reduced to sending to a host on the "directly attached" network
• simple, universal algorithm

Network and Host parts

• Class-based addressing
• Subnetting
• Classless addressing (Classless Inter-Domain Routing, CIDR)

Class-based addressing

• Class A addresses start with a 0 bit, followed by 7 bits of network ID and 24 bits of host ID
• Class B addresses start with 10, followed by 14 bits of network ID and 16 bits of host ID
• Class C addresses start with 110, followed by 21 bits of network ID and 8 bits of host ID
• Problems:
  • Class B is too large (uses too many bits)
  • Class C is too small for a lot of organizations (e.g. ISP, company)
  • don't ever want to renumber, so sign up for largest possible block
  • soon (1994?) "ran out" of class B addresses

Subnetting

• split big network into smaller networks
• use a network mask to identify the network part of the address
• e.g. 255.255.255.192 identifies a subnetwork with 64 host addresses
• from outside network, send to network (identified by class)
• within network, use network mask to find subnetwork/host parts

CIDR

• use network masks to route also from outside of network
• no longer need classes
• a single large network can include smaller networks which can include smaller networks (flexible hierarchy)