Difference between revisions of "IP Classes"

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===Allocation===
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Originally, an IP address was divided into two parts:
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* Network ID: first octet
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* Host ID: last three octets
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This created an upper limit of 256 networks. As the networks began to be allocated, this was soon seen to be inadequate.
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To overcome this limit, different ''classes'' of network were defined, in a system which later became known as [[classful network]]ing.
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Five classes were created (A, B, C, D, and E), three of which (A, B, and C) had different lengths for the network field. The rest of an address was used to identify a host within a network, which meant that each network class had a different maximum number of hosts. Thus there were a few networks with each having many host addresses and numerous networks with each only having a few host addresses.
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Class D was for [[multicast]] addresses and Class E was reserved.
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Around 1993, these classes were replaced with a [[Classless Inter-Domain Routing]] (CIDR) scheme, and the previous scheme was dubbed "classful", by contrast.
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CIDR's primary advantage is to allow re-division of Class-A, -B and -C networks so that smaller (or larger) blocks of addresses may be allocated to various entities (such as [[Internet service provider]]s, or their customers) or local area networks.
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The actual assignment of an address is not arbitrary. The fundamental principle of [[routing]] is that the address of a device encodes information about the device's location within a network. This implies that an address assigned to one part of a network will not function in another part of the network. A hierarchical structure, created by CIDR and overseen by the [[Internet Assigned Numbers Authority]] (IANA) and its [[Regional Internet Registry|Regional Internet Registries]] (RIRs), manages the assignment of Internet addresses worldwide. Each RIR maintains a publicly-searchable [[WHOIS]] database that provides information about IP address assignments; information from these databases plays a central role in numerous tools that attempt to locate IP addresses geographically.
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{| class="wikitable"
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|+ Reserved address blocks
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|-
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! [[Classless Inter-Domain Routing|CIDR]] address block || Description || Reference
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|-
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| 0.0.0.0/8 || Current network (only valid as source address) || RFC 1700
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|-
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| 10.0.0.0/8 || [[Private network]] || RFC 1918
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|-
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| 14.0.0.0/8 || Public data networks (per 2008-02-10, available for use<ref name=net14>[http://www.icann.org/announcements/announcement-2-10feb08.htm ICANN Recovers Large Block of Internet Address Space]</ref>)|| RFC 1700
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|-
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| 127.0.0.0/8 || [[Localhost|Loopback]] || RFC 3330
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|-
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| 128.0.0.0/16 || Reserved (IANA) || RFC 3330
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|-
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| 169.254.0.0/16 || [[Zeroconf|Link-Local]] || RFC 3927
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|-
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| 172.16.0.0/12 || [[Private network]] || RFC 1918
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|-
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| 191.255.0.0/16 || Reserved (IANA) || RFC 3330
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|-
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| 192.0.0.0/24 || Reserved (IANA) || RFC 3330
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|-
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| 192.0.2.0/24 || Documentation and example code || RFC 3330
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|-
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| 192.88.99.0/24 || [[IPv6]] to IPv4 relay || RFC 3068
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|-
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| 192.168.0.0/16 || [[Private network]] || RFC 1918
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|-
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| 198.18.0.0/15 || Network benchmark tests || RFC 2544
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|-
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| 223.255.255.0/24 || Reserved (IANA) || RFC 3330
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|-
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| 224.0.0.0/4 || [[Multicast]]s (former Class D network) || RFC 3171
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|-
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| 240.0.0.0/4 || Reserved (former Class E network) || RFC 1700
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|-
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| 255.255.255.255 || Broadcast ||
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|}
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[[category:IP]][[Category:IPv4]]
 
[[category:IP]][[Category:IPv4]]

Revision as of 12:32, 23 February 2009

Allocation

Originally, an IP address was divided into two parts:

  • Network ID: first octet
  • Host ID: last three octets

This created an upper limit of 256 networks. As the networks began to be allocated, this was soon seen to be inadequate.

To overcome this limit, different classes of network were defined, in a system which later became known as classful networking. Five classes were created (A, B, C, D, and E), three of which (A, B, and C) had different lengths for the network field. The rest of an address was used to identify a host within a network, which meant that each network class had a different maximum number of hosts. Thus there were a few networks with each having many host addresses and numerous networks with each only having a few host addresses. Class D was for multicast addresses and Class E was reserved.

Around 1993, these classes were replaced with a Classless Inter-Domain Routing (CIDR) scheme, and the previous scheme was dubbed "classful", by contrast. CIDR's primary advantage is to allow re-division of Class-A, -B and -C networks so that smaller (or larger) blocks of addresses may be allocated to various entities (such as Internet service providers, or their customers) or local area networks.

The actual assignment of an address is not arbitrary. The fundamental principle of routing is that the address of a device encodes information about the device's location within a network. This implies that an address assigned to one part of a network will not function in another part of the network. A hierarchical structure, created by CIDR and overseen by the Internet Assigned Numbers Authority (IANA) and its Regional Internet Registries (RIRs), manages the assignment of Internet addresses worldwide. Each RIR maintains a publicly-searchable WHOIS database that provides information about IP address assignments; information from these databases plays a central role in numerous tools that attempt to locate IP addresses geographically.

Reserved address blocks
CIDR address block Description Reference
0.0.0.0/8 Current network (only valid as source address) RFC 1700
10.0.0.0/8 Private network RFC 1918
14.0.0.0/8 Public data networks (per 2008-02-10, available for use[1]) RFC 1700
127.0.0.0/8 Loopback RFC 3330
128.0.0.0/16 Reserved (IANA) RFC 3330
169.254.0.0/16 Link-Local RFC 3927
172.16.0.0/12 Private network RFC 1918
191.255.0.0/16 Reserved (IANA) RFC 3330
192.0.0.0/24 Reserved (IANA) RFC 3330
192.0.2.0/24 Documentation and example code RFC 3330
192.88.99.0/24 IPv6 to IPv4 relay RFC 3068
192.168.0.0/16 Private network RFC 1918
198.18.0.0/15 Network benchmark tests RFC 2544
223.255.255.0/24 Reserved (IANA) RFC 3330
224.0.0.0/4 Multicasts (former Class D network) RFC 3171
240.0.0.0/4 Reserved (former Class E network) RFC 1700
255.255.255.255 Broadcast