Federal Internet Law & Policy
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IP Numbers

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A defining characteristic of being on a network is utilization of the network’s address space. On the public telephone network, the address space is telephone numbers set by the North American Numbering Plan. On the Internet, the address space is Internet Protocol Numbers set by IANA. [See Leonard Kleinrock, et. al., Realizing the Information Future: The Internet and Beyond, National Research Council 75 (1994) (comparing addressing on the PSTN and the Internet, stating "The current address spaces of the Internet and the telephone network are a low-level framework suited for naming network and telephone locations and delivering data and voice.")] [See also Will the Real Internet Please Stand Up (concluding that a network is defined by its address space)]

IP numbers are assigned to each Internet host (i.e., computer). The entire collection of IP numbers is managed by the Internet Assigned Number Authority (IANA) under the authority of ICANN. IANA then delegates large blocks of numbers and assignment responsibilities to Regional Internet Registries (RIRs):

  • American Registry of Internet Numbers (ARIN),
  • Asia Pacific Network Information Center (APNic),
  • Réseaux IP Européens Network Coordination Centre, (RIPE NCC)
  • Latin American and Caribbean IP address Regional Registry (Lacnic), and
  • AfriNIC.

    [The RIRs collectively come together as the Number Resource Organization Letter from NRO to ICANN March 23, 2009 confirming NRO's commitment (recognizing) to ICANNPDF; Letter from ICANN to NRO April 17 2009 confirming ICANN's commitment (recognizing) the RIRsPDF]

    ICANN works with the RIRs through the Address Supporting Organization

  • The RIRs then delegate large blocks of numbers to large networks such as Internet backbones, universities, or large corporate networks. Smaller networks or end users get IP numbers from their upstream network providers.

    There were address blocks allocated prior to the current RIR system. These are known as the Legacy Blocks and generally are administered by ARIN, however, the block assignments may not be under ARIN's current contracts and agreements.


  • Joe gets his IP number for his computer from ACME ISP.
  • ACME ISP gets its backbone connection and IP number block from BETA Backbone.
  • Beta Backbone gets its number delegation from ARIN.
  • ARIN gets its numbers from IANA.
  • A packet of data going through the Internet is addressed with the IP number of its destination. The routers through which the packet is transmitted look only at the address (have no regard for the content of the packet) and routes the packet using routing tables determining the best available route. To be visible, a device must be associated with an address; if a device is not associated with an address, it cannot be reached and is not “on network.”

    Now consider that although devices on network are reachable using IP numbers, those numbers are largely known only to the network. Humans are all but unaware of IP numbers. Instead, recognizing years ago that the IP numbers are difficult for the humans to remember, the domain name system (DNS) was developed. A domain name is an alias for an IP number, that is both easier to remember and also can be reassigned from one IP number (one computer at one host) to another IP number (another computer at another host) without any interruption of service.

    Partly out of a belief that the IPv4 set of available numbers is limited and becoming exhausted, IPv6 has been developed and is being deployed.

    Derived From: GAO, Internet Protocol version 6, Federal Agencies Need to Plan for Transition and Manage Security Risks (May 2005)

    The two basic functions of IP include (1) addressing and (2) fragmentation of data, so that information can move across networks. An IP address consists of a fixed sequence of numbers. IPv4 uses a 32-bit address format, which provides approximately 4.3 billion unique IP addresses. Figure 1 provides a conceptual illustration of an IPv4 address.

    By providing a numerical description of the location of networked computers, addresses distinguish one computer from another on the Internet. In some ways, an IP address is like a physical street address. For example, in the physical world, if a letter is going to be sent from one location to another, the contents of the letter must be placed in an envelope that contains addresses for the sender and receiver. Similarly, if data is going to be transmitted across the Internet from a source to a destination, IP addresses must be placed in an IP header. Figure 2 provides a simplified illustration of this concept. In addition to containing the addresses of sender and receiver, the header also contains a series of fields that provide information about what is being transmitted.

    Routing Tables and CIDR

    Derived From: Characteristics and Competitiveness of the Internet Backbone Market , GAO-02-16 p.34 (Oct. 2001).

    Devices known as "routers" send data packets from one network to another by examining the destination IP address of each packet. In its memory, the router contains a "routing table" which contains information specifying all of the IP addresses of other networks. The router compares a packet's destination IP address with the information contained in the routing table to determine the network to which the packet should be sent. In order to ensure that packets from one network can reach any other network, the router must include an entry for each possible network. As more and more network addresses come into use, there is concern about the growth in the number of routing tables entries.

    Historically, IP addresses were organized into three commonly used classes-Classes A, B, and C. For Class A, there are 126 possible network addresses, each with nearly 17 million hosts. Slightly more than 16,000 networks may have a Class B address, each with over 65,000 hosts. Finally, there can be approximately 2 million networks with a Class C address, each with a maximum of 254 host addresses. As the Internet grew, engineers quickly identified the problems associated with exhaustion of class B addresses and the increasing number of Class C address entries in routing tables and developed a solution known as Classless Inter-Domain Routing (CIDR). CIDR treats multiple contiguous Class C addresses as a single block that requires only one entry in a routing table. This method of IP address allocation was adopted for technical efficiency reasons-the number of IP addresses that must be maintained in each router for traffic routing purposes is substantially reduced. However, this method of IP address allocation presents unique problems for smaller ISPs and other entities. If an entity seeking IP addresses cannot utilize a large block of address issued by ARIN, the entity must obtain their addresses from among the allocations made by ARIN to their Internet backbone provider. ISPs and end users with whom we spoke expressed concern about method of IP address allocation.

    [Karrenberg, Development of RIRs] In order to accommodate more address blocks, this was replaced by classless Inter-Domain Routing in 1993.

    The fields in the header are important to the protocol’s second main function: fragmentation of data. IP fragments information by breaking it into manageable parts. Each part has its own header that contains the sender’s address, destination address, and other information that guides it through the Internet to its intended destination. When the various packets arrive at the final destination, they are put back together into their original form.

    IPv4 Address Conservation

    Networks seek to conserve IPv4 Addresses through the following techniques:

  • The use of Network Address Translation devices (NATs) which allow whole networks to be placed behind single IP addresses
  • Classless Interdomain Routing; CIDR
  • Reclaiming unused IPv4 address space
  • See IPv6 Number Migration. Recovering IPv4 Address Space , ICANN Blog (Feb. 6, 2008) ("With help from the Regional Internet Registries, three /8s were returned in 2007 and last month we recovered one more.")



    IP Address Block Allocations and Transfers

    Derived From: Potential Impacts on Communications from IPv4 Exhaustion & IPv6 Transition Robert Cannon, OSP, December 2010 Paper: Word | Acrobat |

    IP address blocks have historically been allocated based on need. 1 The costs involved in receiving an allocation are nominal and are not generally a factor in considering whether to apply for an allocation. 2 The principle requirement has been the ability to demonstrate need for the IP addresses, pursuant to community developed RIR address policy. If an address block was not needed, it would (in theory) be returned; it could not be traded.

    IPv4 conservation has dampened the pace of IPv4 exhaustion. In the early days of the Internet when the US dominated Internet use, some US firms received large IPv4 block allocations; some of these entities have returned unused IPv4 address resources to Internet number registries. 3 While these conservation efforts have helped, they have merely delayed IPv4 exhaustion without solving the long-term problem. 4

    One proposal has been to allow transfers and trade of IP blocks (instead of returning unused resources to the RIRs). 5 This could create an incentive for holders of underutilized IP address blocks to sell them to parties that would put them to more productive use. 6 Transferring IPv4 number allocations would enable new entrants to acquire assignments of IP number resources that are not subordinate to a legacy stakeholder. 7 It would also take pressure off during the transition period, permitting networks to continue to expand, and allowing those engaged in the transition additional time to resolve any transition issues encountered. 8 Two RIRs have policies that permit transfers of IP address block assignments under certain conditions. 9

    The addresses transferred are just numbers. For them to be valuable, they must be routable. The routability of the numbers could be unstable if an RIR does not authenticate the transfer, if conflicting claims to the numbers arise, or if there is any other corruption in the integrity of a unique number assignment to network. 10

    Unauthorized transfers could create an issue of the RIR not having a direct relationship with, and knowledge of, the transferee, and thus be unable to maintain accurate address assignment records along with associated contact information. 11

    There is concern that the scarcity of IPv4 numbers will result in IPv4 number hijacking where addresses are utilized by someone other than the assignee of record. 12

    Hijacking IP addresses for purposes of spam may be a violation of federal law. According to 18 USC 1037(a)(5): " Whoever, in or affecting interstate or foreign commerce, knowingly-... (5) falsely represents oneself to be the registrant or the legitimate successor in interest to the registrant of 5 or more Internet Protocol addresses, and intentionally initiates the transmission of multiple commercial electronic mail messages from such addresses, .... or conspires to do so, shall be punished as provided in subsection (b)." See Can Spam Act.

    Ray Plzak, IP Address Hijacking: An ARIN Perspective (Nov. 2003) (PDF)

    The resulting lack of accurate address information also has significant implications for law enforcement and global anti-cybercrime efforts.

    Finally, there is also concern about the impact of address transfers on the routing table and fragmentation.

    1 Geoff Huston, IPv4 Address Report.

    2See ARIN Number Resource Policy Manual, Sec. 4.2 Allocation to ISPs (Jan. 13, 2010).

    3 Lljitsch van Beinjnum, Everything You Need to Know About IPv6, Ars Technica (Mar. 7, 2007) ("For instance, IBM, Xerox, HP, DEC, Apple and MIT all received "class A" address blocks of nearly 17 million addresses. (So HP, which acquired DEC, has more than 33 million addresses.)"); Geoff Huston, IPv4 Address Report ("Unneeded addresses are to be passed back to the registry. "); Recovering IPv4 Address Space, ICANN Blog (Feb. 6, 2008) ("With help from the Regional Internet Registries, three /8s were returned in 2007 and last month we recovered one more.")

    4 Lljitsch van Beinjnum, Everything You Need to Know About IPv6, Ars Technica (Mar. 7, 2007) (such efforts only buys us a few more years).

    5See Milton Mueller, Scarcity in IPv4 Addresses: IPv4 Address Transfer Markets and the Regional Internet Address Registries, IGP (July 20, 2008); OECD Study: Economic considerations in the management of IPv4 and in the deployment of IPv6, p. 26 (May 2008); Huston, G., IPv4 address transfers, proposed to APNIC on 26 July 2007 ; Titley, N. and van Mook, R., Enabling methods for reallocation of IPv4 resources, (Oct. 23, 2007); Dan Campbell, Comments on an IP Address Trading Market , CIRCLEID (Feb. 15, 2008).

    6See Next Generation Internet: IPv4 Address Exhaustion, Mitigation Strategies and Implications for the US, IEEE-USA White Paper, p. 11 (2009); Communication From the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions, Advancing the Internet: Action Plan for the Deployment of Internet Protocol version 6 (IPv6) in Europe, p. 4 (May 27, 2008). See RIPE NCC IPv4 Address Allocation and Assignment Policies for the RIPE NCC Service Region, Sec. 5.5 Feb. 2010.

    7 OECD Study: Economic considerations in the management of IPv4 and in the deployment of IPv6, p. 27 (May 2008).

    8 Milton Mueller, Scarcity in IPv4 Addresses: IPv4 Address Transfer Markets and the Regional Internet Address Registries, IGP p. 17 (July 20, 2008) ( " The transition could turn out to be more complicated, costly and difficult than anticipated, and we don't know how long it will last. If we try to use an address shortage to force ISPs into making the transition before they are ready, we could develop damaging gaps in connectivity due to shortages of address resources and compatibility problems. " ).

    9See ARIN Number Resource Policy Manual, Sec. 4.2.3 Reassigning Address Space to Customers (Sept. 2010); IPv4 Address Allocation and Assignment Policies for RIPE NCC Service Region, Sec. 5.5 Transfers of Allocations (Oct. 2010).

    10See OECD Study: Economic considerations in the management of IPv4 and in the deployment of IPv6, p. 27 (May 2008); Ray Plzak, IP Address Hijacking: An ARIN Perspective (Nov. 2003) (PDF).

    11 See Dan Campbell, Comments on an IP Address Trading Market, CIRCLEID (Feb. 15, 2008); IPv6 in Canada: Final Report and Recommendations of the ISACC IPv6 Task Group (IITG), IITG Final Report to ISACC, ISACC-10-42200, p. 16 (Mar. 16, 2010) (" Unclear ownership of some IPv4 addresses plus a lack of tools to block wrong addresses could lead to instability of the routing system ") .

    12 Ray Plzak, IP Address Hijacking: An ARIN Perspective (Nov. 2003).

    "Stanford University , which was originally allocated nearly 17 million IP addresses , restructured its network in 2000 and gave back a Class A address block equal to approximately 16 million IP addresses. See Carolyn Marsan, “Stanford Move Rekindles ‘Net Address Debate,’” NWFusion (Jan. 24, 2000), NTIA Report: Technical and Economic Assessment of IPv6, p. 12 n 47 July 2004

    Notes :: IP Numbers as Property

  • Ripe NCC, “Global Distribution of IP-Addresses,”.
  • RIPE NCC IPv4 Address Allocation and Assignment Policies for the RIPE NCC Service Region, Sec. 5.5 Feb. 2010
  • Depository Inc. Internet Registry
  • Definitions

    Internet Control Message Protocol: runs on top of IP. in parallel with TCP and UDP. Triggered when packets encounters errors, such as time exceeded or destination unreachable. ICMP packet sent to source IP address with error message information.

    IP Number Under the current Internet Protocol system, each machine connected to an Internet Protocol ("IP") network is addressed using a 32 bit number, the IP address.  these addresses are usually written in "dotted quad" notation, as a series of four 8 bit numbers, written in decimal and separated by periods.  for example, an IP address might appear as  Many machines have more than one IP address.  For example, a machine hosting multiple websites often has an IP address for each website it hosts.  Other times, a pool of IP addresses is shared between a number of machines e.g. on a dynamic IP dialup connection such as the Prodigy Internet Service, a subscriber's machine will be allocated a different IP address each time the subscriber connects.   -- British Telecom v. Prodigy, OO Civ 9451, Memorandum and Order Granting Summary Judgment (SDNY August 22, 2002) (citations omitted).

    Public IP Address Space: This term is not formally defined (that I know of). It is the address space that is not the reserved private IP address space (in other words, defined as what it is not). Public IP addresses are routable on the Internet. Unlike private addresses, public addresses are globally unique. [Microsoft Public and Private Addresses]

    Private IP Address Space: An allocation of IP addresses intended to be utilized on a private network for internal routing and not to be announced or routed to the public Internet. Since the address block is not announced and routed, it can be re-used over and over inside networks without creating an addressing conflict (something like PBX in the telephony world). Private addresses are used behind NATs.




  • IANA
  • IPv4 Address Space Allocations
  • IPv6 Top Level Aggregation Identifier Assignments
  • ASNs Autonomous System Numbers
  • Geoff Huston :: IPv4 Report :: IPv6 Report ::
  • USG :: NIST :: CSD :: IPSec Project
  • Liability Based on IP Numbers

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