Looking up Internet Addresses

1
Looking up Internet Addresses

• Chapter 6

2
IP Addresses

• IPv4
• Ex 139.124.12.14
• (0.0.0.0 to 255.255.255.255)
• IPv6
• Ex Address of www.ipv6.com.cn
  20012502ff2102508bfffede67c8
• In a mixed of IPv4 IPv6 networks, the last four
  bytes are written in v4
• Ex fedcba982ff210118.64.70.15
• IPv6 is supported by Java 1.4 later. Java 1.3
  earlier support only 4 byte address.

3
Domain Name System (DNS)

• People can remember only 7 digits (phone numbers
  split into 3 digits-4digits)
• DNS associates hostnames that people can remember
  to their IP addresses.
• Ex www. UNB.CA to lt gt
• Except computers that do not have permanent IP
  address (most PCs ) since they are not used as
  serversdont need a name since nobody will refer
  to them
• Some machines have multiple names. www.xx refers
  to a website not a machine
• Website can be easily moved to another machine
  URLs around the world need not be updated.

4
DNS cont..

• Sometimes one name maps to a number of IP
  addresses. Then the DNS server must randomly
  select machines to respond to each request. Used
  in high traffic web sites, where it splits the
  load across multiple systems.
• Ex www.oreilly.com is actually in 2 machines
  208.201.239.36 208.201.239.37
• Each computer connected to Internet must have
  access to a machine called Domain Name Server
  (locally)usually a unix box running special DNS
  software that knows the mapping between different
  hostnames IP addresses.
• If a client requests for the IP address outside
  the domain, the local DNS server asks a remote
  DNS server relays the answer to the request!

5
The InetAddress Class

• Java.net.InetAddress class in java is a
  high-level representation of an IP address, both
  IPv4 v6.
• It is used by most other network classes, such as
  Socket, ServerSocket, URL, DatagramSocket, and
  many more..
• Includes both a hostname IP address

6
Creating new InetAddress objects

• InetAddress class has three static methods that
  return suitably initialized InetAddress objects.
• Public static InetAddress getByName (String
  hostName)
• throws UnknownHostException
• Public static InetAddress getAllByName (String
  hostName)
• throws UnknownHostException
• Public static InetAddress getLocalHost ()
• throws UnknownHostException