Editing Subnet (section)

===Special addresses and subnets===
IPv4 uses specially designated address formats to facilitate recognition of special address functionality. The first and the last subnets obtained by subnetting a larger network have traditionally had a special designation and, early on, special usage implications.<ref>{{cite web
 |url          = http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml
 |title        = Document ID 13711 - Subnet Zero and the All-Ones Subnet
 |publisher    = [[Cisco Systems]]
 |date         = 2005-08-10
 |access-date  = 2010-04-25
 |quote        = Traditionally, it was strongly recommended that subnet zero and the all-ones subnet not be used for addressing. [...] Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use.
 |archive-date = 2014-02-09
 |archive-url  = https://web.archive.org/web/20140209020855/http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml
 |url-status   = live
}}</ref> In addition, IPv4 uses the ''all ones'' host address, i.e. the last address within a network, for broadcast transmission to all hosts on the link.

{{anchor|subnet zero}}{{anchor|all-ones subnet}}
The first subnet obtained from subnetting a larger network has all bits in the subnet bit group set to zero. It is therefore called ''subnet zero''.<ref>{{cite web
 |url          = http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml
 |title        = Document ID 13711 - Subnet Zero and the All-Ones Subnet
 |publisher    = [[Cisco Systems]]
 |date         = 2005-08-10
 |access-date  = 2010-04-23
 |quote        = the first [...] subnet[...], known as subnet zero
 |archive-date = 2014-02-09
 |archive-url  = https://web.archive.org/web/20140209020855/http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml
 |url-status   = live
}}</ref> The last subnet obtained from subnetting a larger network has all bits in the subnet bit group set to one. It is therefore called the ''all-ones subnet''.<ref>{{cite web
 |url          = http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml
 |title        = Document ID 13711 - Subnet Zero and the All-Ones Subnet
 |publisher    = [[Cisco Systems]]
 |date         = 2005-08-10
 |access-date  = 2010-04-23
 |quote        = [...] the last subnet[...], known as [...] the all-ones subnet
 |archive-date = 2014-02-09
 |archive-url  = https://web.archive.org/web/20140209020855/http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml
 |url-status   = live
}}</ref>

The IETF originally discouraged the production use of these two subnets. When the prefix length is not available, the larger network and the first subnet have the same address, which may lead to confusion. Similar confusion is possible with the broadcast address at the end of the last subnet. Therefore, reserving the subnet values consisting of all zeros and all ones on the public Internet was recommended,<ref name="rfc950p6">{{Cite IETF
    | rfc        = 950
    | publisher  = [[IETF]]
    | author     = Jeffrey Mogul
    | author2    = Jon Postel
    | authorlink2= Jon Postel
    | title      = Internet Standard Subnetting Procedure
    | page       = 6
    | date       = August 1985
    | quote      = It is useful to preserve and extend the interpretation of these special addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets.
}}</ref> reducing the number of available subnets by two for each subnetting. This inefficiency was removed, and the practice was declared obsolete in 1995 and is only relevant when dealing with legacy equipment.<ref name="rfc1878">{{Cite IETF
    | rfc       = 1878
    | publisher = [[IETF]]
    | title     = Variable Length Subnet Table For IPv4
    | author    = Troy Pummill
    | author2   = Bill Manning
    | date      = December 1995
    | quote     = This practice is obsolete! Modern software will be able to utilize all definable networks.
}} (Informational RFC, demoted to category ''Historic'')</ref>

Although the all-zeros and the all-ones host values are reserved for the network address of the subnet and its [[broadcast address]], respectively, in systems using CIDR all subnets are available in a subdivided network. For example, a {{IPaddr||24}} network can be divided into sixteen usable {{IPaddr||28}} networks. Each broadcast address, i.e. {{IPaddr|*.15}}, {{IPaddr|*.31}}, …, {{IPaddr|*.255}}, reduces only the host count in each subnets.