ipaddFAQ.html / James Speed Hensinger / jhensing@bcr.org / Updated 4/2/98
What is an IP Address?
All devices connected to the Internet are identified by a unique IP number. The IP stands for Internet Protocol,
which is the name for a standardized identification scheme. It is via IP numbers that different computers know
how to identify each other. You may know a computer by its name (more correctly its domain name), but names
aren't the real way different computers are identified. When you use a browser to surf to a name, somewhere
along the communication path, the name is translated into the IP number for that machine. This is done by a
Domain Name Server, which is a computer that has lookup tables for domain names and their relevant IP
numbers. When organizations apply for a domain name (such as bcr.org) that they want to register through their
Internet Provider they are usually assigned a range of numbers that they, in turn, assign to their computers.
What does an IP Address look like?
IP numbers take the form of four groups of a maximum of three digits each separated by periods (full stops).
The periods are usually verbalized as "dot".
Example: 199.045.145.123
This is equivalent to: 199.45.145.123
What is a range of IP Addresses?
Any three digit group may have a value ranging from 0 through 255. If your organization "owns" the last three
digit group, a Class C license, then you "own" 256 unique IP numbers. A class B license owns all the numbers
in the next to last and last groups. This means that a class B license has 256 X 256 = 65,536 numbers.
All of these format examples of a Class C license mean the same thing:
What is the difference between static and dynamic IP Addressing?
There are actually two ways of assigning IP numbers, static and dynamic. Static IP numbers are permanently
assigned to a computer. Dynamic IP numbers are only assigned to a computer at the time it connects with the
Internet. Dynamic IP numbers can change every time the computer reconnects to the Internet. Dynamic IP
numbers are used to maximize the use of the numbers available to an organization or an Internet Provider.
What is IP filtering?
Many Web-based services can regulate customer access by identifying their customers via the IP number the
customer's computer attaches to every message. These services can avoid the inconvenience of requiring the
customer to enter a username and password to gain access. This is extremely useful for services such as
online encyclopedias that are purchased for an entire building or campus. Only static IP numbers can be used
in this way.
What is the effect of a firewall or proxy server?
Some organizations install special security systems on their Internet access called "firewalls" or "proxy servers".
One effect of doing this is that the IP number that is used on individual computers may not be projected onto the
Web. Instead the firewall or proxy server acts as a forwarding service and attaches a single IP number to the
message that is used on all messages from all of the computers behind the firewall or proxy server. For this
reason, it can be important to know the IP number of the firewall or proxy server if IP recognition is needed.
IP Network Numbers - Class A
by John Crossley - E-Mail: jgc@americansouthwest.net
Class A networks (0.x.x.x to 127.x.x.x) have an 8-bit network prefix with a 32-bit host number, and are also referred to as /8 networks.
The first bit is set to 0 to identify the network as a Class A type; this results in 128 (27) possible Class A networks, but those with prefix 0 and 127 are used for special functions and the upper half (with prefixes above 64) are reserved for future use.
This information was updated November 1999.
0 Internet Assigned Numbers Authority (IANA) (NET-RESERVED-1)
1 Internet Assigned Numbers Authority (IANA-ARIN) (NET-RESERVED-9)
2 Internet Assigned Numbers Authority (IANA) (NET-RESERVED-2)
3 General Electric Company, NJ (NET-GE-INTERNET)
4 BBN Planet, MA (NET-SATNET)
5 Internet Assigned Numbers Authority (IANA) (NET-RESERVED-5)
6 Army Information Systems Center - USAISC, Yuma Proving Ground, AZ (NET-YPG-NET)
7 Defense Information Systems Agency, VA (NET-DISANET2)
8 Bolt Beranek and Newman Inc., MA (NET-BBN-NET-TEMP)
9 IBM Corporation, NY (NET-IBM)
10 Internet Assigned Numbers Authority (IANA) (RESERVED-6)
This is reserved for private internet use, without connection to the global network
11 DoD Intel Information Systems, Defense Intelligence Agency, Washington DC (NET-DODIIS)
12 AT&T (NET-ATT)
13 Xerox Palo Alto Research Center, CA (NET-XEROX-NET)
14 Public Data Network, US (NET-PDN)
15 Hewlett-Packard Company, CA (NET-HP-INTERNET)
16 Digital Equipment Corporation (NET-DEC-INTERNET)
17 Apple Computer Inc., CA (NET-APPLE-WWNET)
18 Massachusetts Institute of Technology, MA (NET-MIT-TEMP)
19 Ford Motor Company, MI (NET-FINET)
20 Computer Sciences Corporation, VA (NET-CSC)
21 US Defense Information Systems Agency (DDN-RVN), VA (NET-DDN-RVN)
22 Defense Information Systems Agency, Washington DC (NET-DISNET)
23 IANA (reserved) (NET-DDN-TC-NET) (RESERVED-23)
24 US Cable Networks
25 Royal Signals and Radar Establishment, UK (NET-RSRE-EXP)
26 Defense Information Systems Agency, VA (NET-MILNET)
27 Internet Assigned Numbers Authority (IANA) (RESERVED-10)
28 ARPA DSI JPO, VA (NET-DSI-NORTH)
29 Defense Information Systems Agency, Washington DC (NET-MILX25-TEMP)
30 Defense Information Systems Agency, Washington DC (NET-ARPAX25-TEMP)
31 IANA (RESERVED-12)
32 IBM Global Services, NH (NETBLK-IBMGLOBALSERV)
33 DLA Systems Automation Center, OH (NET-DCMC)
34 Halliburton Company, TX (NET-HALLIBURTON)
35 MERIT Network Inc., MI (NET-MERIT)
36 Stanford University, CA (NET-SU-NET-TEMP)
37 Internet Assigned Numbers Authority (IANA) (RESERVED-37A)
38 Performance Systems International, VA (NET-PSINETA)
39 Internet Assigned Numbers Authority (IANA) (RESERVED-39A)
40 Eli Lilly and Company, IN (NET-LILLY-NET)
41 Internet Assigned Numbers Authority (RESERVED-41A)
42 Internet Assigned Numbers Authority (IANA) (RESERVED) (NET-RESERVED-42)
43 Japan Inet, Japan (NET-JAPAN-A)
44 Amateur Radio Digital Communications (NET-AMPRNET)
45 Interop Show Network, CA (NET-SHOWNETA)
46 Bolt Beranek and Newman Inc., MA (NET-BBNNET)
47 Bell-Northern Research, Canada (NET-BNR)
48 Prudential Securities Inc., NY (NET-PRUBACHE)
49 Joint Tactical Command, Control, and Communications Agency, AZ (NET-JITCNET1)
50 Joint Tactical Command, Control, and Communications Agency, AZ (NET-JITCNET2)
51 Department of Social Security of UK, England (NET-ITSANET)
52 E.I. duPont de Nemours and Co., Inc., DE (NET-DUPONT1)
53 cap debis ccs (Mercedes Benz), Germany (NET-DB-NET2)
54 Merck and Co., Inc., NJ (NET-MERCK2)
55 Army National Guard Bureau, VA (NET-RCAS2)
56 U.S. Postal Service, NC (NET-USPS1)
57 SITA - Societe Internationale de Telecommunications Aeronautiques, France (NET-SITA2)
58 Internet Assigned Numbers Authority (IANA) (RESERVED) (NET-RESERVED-58)
59 Internet Assigned Numbers Authority (IANA) (RESERVED) (NET-RESERVED-59)
60 Internet Assigned Numbers Authority (IANA) (RESERVED) (NET-RESERVED-60)
61 Asia Pacific Network Information Center (NETBLK-APNIC2)
62 European Regional Internet Registry/RIPE NCC (NETBLK-RIPE-C3)
63 InterNIC Registration (NETBLK-INTERNIC-2)
64 - 95 Internet Assigned Numbers Authority (IANA) (RESERVED-7)
96 - 126 Internet Assigned Numbers Authority (IANA) (RESERVED-8)
127 Internet Assigned Numbers Authority (IANA) (LOOPBACK)