CS 210 Web Page Design

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CS 210 Web Page Design
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The Internet

• The Internet is a network of networks that are
  spread all over the world
• Networks that are geographically close are called
  Local Area Networks (LANs)
• Often in the same building
• The universitys network is a prime example
• The Internet is a largely heterarchical network
  containing many individual LANs

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The Internet

• A heterarchical network contains many nodes that
  are interconnected.
• A hierarchical network contains a tree-like
  structure where some nodes are superior to others.

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The Internet
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The Internet

• A characteristic of a heterarchical network is
  that it is a robust network.
• If some nodes are removed, data can still be sent
  between nodes
• Hierarchical networks do not lend themselves to
  robustness
• The Internet also has dynamic routing, where the
  route of the data is determined at the time of
  transmission based on current network conditions.

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Host Machines

• Each computer on the Internet is a host machine.
• Each computer has a unique Internet Protocol (IP)
  address, such as 124.110.24.1
• Some computers have a permanent IP address
• Some computers borrow an IP address while they
  are connected to the Internet
• An IP address is not human-friendly

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Host Machines

• The IP address for most host machines are mapped
  to a Domain Name Service (DNS) address in order
  to be more people-friendly
• The DNS address consists of a host name followed
  by a domain name
• Example DNS Address mail.yahoo.com
• Host Name is mail
• Domain Name is yahoo.com

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Host Names

• Each domain name consists of
• Institutional site name
• Top Level Domain name (TLD)
• Example cs.umass.edu
• cs.umass is the Dept. of Computer Science at the
  University of Massachusetts at Amherst
• edu refers to an educational site

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TLDs

• Examples of TLDs include
• .com a commercial organization
• .edu a US educational site
• .net a network site
• .au Australia
• .fr France
• .hk Hong Kong
• .es Spain

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Host Machines

• New TLDs have been added as the original set
  became overloaded
• While each machine has a unique IP address, it
  can have multiple DNS addresses (called aliases)
• Anyone can register a DNS address
• When you type in a DNS address, a domain name
  server translates it into an IP address.

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Speed and BottleNecks

• When you go online, your computer exchanges data
  with other computers.
• The transfer of data is measured in bits per
  second (bps) bandwidth
• Your data transfer rate is determined by
• The type of connection (e.g. dial-up)
• The traffic over the network
• The bottleneck is the part of your connection
  that slows the data rate during transfer

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Client/Server Model

• Clients and servers are host machines
• A client is the host machine that requests
  information from the server
• The server is a resource that provides a service
  for (many) clients
• The client/server interaction is the foundation
  for all Internet communication

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Web Browsers

• The World Wide Web has become a popular means of
  accessing information and services.
• The Web and the Internet are not the same.
• A Web browser is the software necessary to view
  information.
• Some browsers are integrated into other software,
  such as an email client

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DNS

• Short for Domain Name System (or Service or
  Server), an Internet service that translates
  domain names into IP addresses. For example, the
  domain name www.example.com might translate to
  198.105.232.4.
• The DNS system is, in fact, its own network. If
  one DNS server doesn't know how to translate a
  particular domain name, it asks another one, and
  so on, until the correct IP address is returned.

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DNS

• It also lists mail exchange servers accepting
  e-mail for each domain.
• DNS is an essential component of contemporary
  Internet use.

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DNS history

• Originally, each computer on the network
  retrieved a file called HOSTS.TXT from SRI which
  mapped an address to a name.
• The Hosts file still exists on most modern
  operating systems, either by default or through
  configuration, and allows users to specify an IP
  address to use for a hostname without checking
  the DNS
• Problem every time a given computer's address
  changed, every computer that wanted to
  communicate with it would need an update to its
  Hosts file.

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DNS

• Need a more scalable system.
• Store the change in a hosts address in a single
  place.
• All other hosts would learn about the change
  dynamically through a notification system.

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DNS

• The domain name space is a tree-like structure.
• Each node or leaf in the tree contains a resource
  record.
• This record holds the information for that domain
  name.

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DNS

• When a name needs to be transformed to its
  corresponding IP address, a resolver is used.
• This resolver knows how to communicate with these
  name servers through DNS requests and responses.
• Resolving usually means recursing through
  multiple name servers until the answer is found.

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Address Resolution

• 1) the local system is pre-configured with the
  known addresses of the root servers
• These should be updated periodically from a
  reliable source.
• 2) one of these root servers should be queried
  for the next level down of the domain name. ( top
  level domain )

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Address Resolution

• 3) query this second server for the address of a
  DNS server with detailed knowledge of the
  second-level domain
• 4) Repeat step 3 until the address needed is
  found and returned.

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Problems

• This form of address resolution can and will
  place a heavy work load on those root servers.
• Solution Caching

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Caching

• Store the final result and intermediate results
  in a cache for later use.
• When you want to visit the same web site, the
  cache can be used to find the resolved address
  faster than going through the name servers.
• It also reduces the burden placed upon these root
  servers.