Java Glossary : IP

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IP
Internet Protocol. The lowest level protocol of the Internet upon which everything else is based. It simply sends packets, with no checks that they arrived and no retransmission. Built on top of this are the UDP and TCP/IP protocols. Every machine on the Internet has a unique 32-bit address called its IP. Parts of the Internet have converted to 128-bit addresses called IPv6 (Internet Protocol Version 6). A standard IPv4 (Internet Protocol Version 4) IP is written as a dotted quad e.g. 24.87.56.253, four numbers 0..255. Sometimes a dotted quad such as 64.251.89.39 is written as a giant 32-bit unsigned decimal number e.g. [1090214183] or in hex [40FB5927]. For example can get at my website by DNS name or by IP number with the following URLs: http://mindprod.com or http://24.87.56.253.

Whois

There are five common questions about domain names and IPs you may have:
  1. Who owns this IP?
  2. I know the domain name. What IP does it map so I can put an entry in my hosts. file.
  3. Who owns this domain name?
  4. Where on the planet is this IP broadcasting from?
  5. What is my face IP?
For the first question see IP lookup services

For the second and third questions use one of these whois services:

For the fourth question, you need an IP locator: For the fifth question you need a face IP identifier: You might think it sufficient for to check if a domain name is taken to simply point your browser to www.xxxxx.com and see if you get a 404 not found error. However, the name may be taken, with no webserver set up, so the name would appear to be free when it was actually taken.

Face IP

To find out your face IP, the IP that outside world (beyond your firewall) thinks your IP and domain name is, try The face ip is different from the IP of your computer on the LAN. An IP of a computer on a LAN is typically something like 192.168.2.7. The router has two ips, one it talks to the computers on the lan often 192.168.2.1 and one it uses to talk to the outside world, dynamically assigned by the ISP using DHCP.

Displaying an IP

public class DottedQuad
   {
   /**
    * display an IP as a dotted quad xxx.xxx.xxx.xxx
    */
   public static String dottedQuad ( int ip )
      {
      StringBuffer sb = new StringBuffer( 15 );
      for ( int shift=24; shift >0; shift-=8 )
         {
         // process 3 bytes, from high order byte down.
         sb.append( Integer.toString( (ip >>> shift) & 0xff ));
         sb.append('.');
         }
      sb.append( Integer.toString( ip & 0xff ) );
      return sb.toString();
      }

   /**
    * test harness
    *
    * @param args not used
    */
   public static void main ( String[] args )
      {
      System.out.println( dottedQuad( 0x01ff0010 ) );
      }
   }

IP bit manipulation tricks

// IP bit tricks
// compute an n bit subnet mask, all 1s except for last n bits.
int subnet = ~( ( 1 << n ) - 1 );

// generate a list of possible local IP addresses, e.g.
// 192.168.0.0 .. 192.168.255.255

int base = (192 << 24) | (168 << 16);
for ( int i=0; i<0xffff; i++ )
   {
   int sample = base | i;
   System.out.println( DottedQuad.dottedQuad( sample ) );
   }

IPV6

The new IP standard is IPv6. It uses 128-bit address. When presented to humans, these 16-byte IPs are written in hex in 8 groups of 4 hex digits like this 1080:0:0:0:8:800:200C:41FA. Lead zeros on a group may be left off.

Determining your own IP Address

In Java you can determine the IP of the machine running the Java application or Applet with:


view

Unfortunately, unsigned Applets will be told the hostname is localhost/127.0.0.1 since such nosiness is considered a security breach. Signed Applets will be told the truth. Further, if you are behind a router or firewall, you will just get your local LAN address, e.g. [192.168.2.34], not the facing address you are known by out on the Internet via your proxy.

In JSP, the following methods may be useful: request.getServerName(), request.getServerPort() and request.getRequestURI()

If you are writing a Applet, you can find out the name of the server indirectly via Applet.getCodeBase. You have to use the URL.getHost method to pull out just the part you want.

If your webserver supports SSI you can get such information via the REMOTE_HOST (URL) REMOTE_ADDR (IP) SERVER_NAME (URL) and SERVER_ADDR (IP) directives.

In Windows, you can use the utility ipconfig /all to display information such as IP, subnet mask, DNS Server IP, host name, physical ethernet 48-bit MAC address, gateway IP and DHCP server IP. In older versions of Windows there is a similar utility called winipconfig. You can spawn this utility from Java with the exec function, and programmatically scan through the output to find what you need. Alternatively, you need to write platform specific JNI code to grab additional information, such as DNS servers, gateways, masks, etc.

There are various utilities that periodically probe to find out your IP address, then notify you if your face IP changes, and also notify the DynDns server.

Finding Out Someone Else's IP

You can find out the name or IP of an arbitrary website given either its name or IP with:


view

You can use

Obviously you must be online to the Internet for these calls to work, and you need a functioning DNS (Domain Name Service lookup) server. I have heard reports that if either of these conditions are not met, instead of getting an exception, you just get 0.0.0.0 as the IP of every website.

If you are behind a firewall you may not have access to DNS or reverse DNS lookup. You might experiment by putting the DNS-IP pair in your Windows or NT HOSTS file, or working purely with IPs. Sometimes the receiver requires a DNS name, not just a pure IP. In that case you can provide one with URLConnection.setRequestProperty().

I know of no way to discover your DNS server IPs without resorting to JNI.

Unfortunately, finding out your server's IP will trip the sandbox security mechanism in an Applet. You could sign the Applet, or you can dance around the problem this way: you get the server name from getCodeBase().getServer(), open a Socket to port 80 and retrieve the local IP address from that Socket. If getServer() returns null, the Applet is running locally, and you could just use localhost namely 127.0.0.1 as the IP address. In Linux you could use localhost@localdomain.

You can find out who owns a domain name at http://www.networksolutions.com/cgi-bin/whois/whois. Unfortunately, you can no longer find out who owns an IP. For that you need arin.net.

If you need finer control than InetAddress gives you, e.g. the ability to select the DNS server, try DNSJava.

How Are IP's Assigned

Websites have a permanently assigned IP. IANA manages assigning them through sort of wholesale distributor system. For dialup, PPP temporarily assigns you an IP that stays constant until you hang up. For workstations with permanent connections, DHCP automatically assigns you a temporary IP number using a discoverable IP number server. With the explosive growth of the Internet, we are running out of IP numbers, and will at some point have to go IPv6 protocol with more address bits.

For computers not directly attached to the Internet, (e.g. via a proxy server), or on a LAN that has Internet access, but without individual station access, or on an isolated LAN, you don't need universally unique IP numbers. You can use the ones reserved for local private LAN use. You don't need to get anyone's permission to use them such as 192.168.0.1 .. 192.168.255.254

IP Addresses
Address
Class		 IP Range Format
A large 0.0.0.0 (null)

1.0.0.1 .. 9.255.255.254

10.0.0.1 .. 10.255.255.254
(any private LAN)

11.0.0.1 .. 126.255.255.254

127.0.0.1 (localhost)
0
1 bit 		 Network
7 bits		 Local address
24 bits
B medium 128.0.0.1 .. 172.15.255.254

172.16.0.1 .. 172.31.255.254
(any private LAN)

172.32.0.1 .. 191.255.255.254
10
2 bits		 Network
14 bits		 Local address
16 bits
C small 192.0.0.1 .. 192.167.255.254

192.168.0.1 .. 192.168.255.254
(any private LAN)

192.169.0.1 .. 223.255.255.254
110
3 bits		 Network
21 bits		 Local address
8 bits

One of the rather terrifying things about the way the Internet is designed is that there is nothing to stop people from configuring their machines with IPs they have no business using. It becomes trivially easy to steal unused or rarely used IPs, to spoof someone else's identity, or to spy on someone else's traffic. The Internet protocols were not designed to deal with malicious teenage hackers, only gentlemanly academics. If engineers built the way computer scientists do, the first woodpecker than came along would destroy civilisation.

How the IP Protocol Works

The IP protocol can best be understood by looking at the header that goes on each packet. The IP packet header is a bit like the baggage tag you attach to your suitcase before entrusting it to the airline:

IPv4 Packet Header
Field Size in bits Purpose
version 4 4 for IP version 4. This allows IPv4 and IPv6 packets to be mixed.
hlen 4 size of packet header measured in 32-bit chunks.
total length 16 total size of packet header plus data payload measured in 8-bit chunks (aka bytes or octets). The means the maximum size of a packet including data is 64K. This means the payload of data is at most 65,515 bytes.
time to live 8 how many more hops/seconds this packet has to live before being discarded as lost or hopelessly late. On each leg of this packet's journey, this field gets decremented by 1 or the number of seconds of delay at that hop if the delay was greater than one second.
header checksum 16 digest of just the header to ensure the header has not been scambled in transmission. It has to be recomputed at each hop to account for the new value of the time to live field. Note that the body of the packet is not protected by checksum. That has to be done at a higher level, e.g. UDP or TCP/IP.
source IP 32 Who sent the packet. When it finally arrives at its destination the receiver will know who it was from.
destination IP 32 Where the packet is going. On each leg of its journey the routing computer uses this to get the packet a little closer to its final destination.
other stuff 48 Miscellaneous fields. See details.

Everything is in big endian byte order.

The sender tosses his packet of data into the Internet, with the header. Computers on the net pass the packet along hoping to get it closer and closer to its destination. Perhaps a dozen hops later it finally arrives. Perhaps it doesn't. There is nothing in IP protocol to guarantee safe arrival.

IP protocol does not sound too promising. Unlikely you will use IP protocol directly. UDP ( via Java classes DatagramPacket and DatagramSocket) and TCP/IP (via Java classes Socket and ServerSocket) protocols are built on top of IP, and FTP and HTTP (via Java class HttpURLConnection) are built on top of TCP/IP.

MaxMind sells code to obtain the country, region, city, latitude, and longitude of any IP address. This would often be an ISP rather than the actual company responsible for content.

book_coverInternetworking with TCP/IP Vol.1: Principles, Protocols, and Architecture, 4th edition
0-13-018380-6
Douglas E. Comer
How IP, UDP and TCP/IP protocols work. How the domain name to IP translation DNS information in propagated.
amazon.com Barnes and Noble
amazon.ca chapters
amazon.co.uk amazon.de

datagram ¤ DHCP ¤ DNS ¤ domain names ¤ DynDNS: three free services of interest to people who host webservers on their home machines ¤ face IP ¤ gateway ¤ hosts ¤ IPv6 ¤ localhost ¤ masking ¤ PPP ¤ subnet mask ¤ TCP/IP ¤ TweakDUN ¤ UDP ¤ whois

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