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{{Short description|Method for IP address allocation and routing}}
{{Redirect|CIDR}}
{{Copy edit |reason=direct links to RFCs (using {{tl|IETF RFC}} that its documentation says is not for that usage) rather than correctly citing these RFCs |date=August 2025}}
{{Use American English |date=August 2025}}
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| style="text-align:right;" |256 A
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| Entire IPv4 Internet, [[default route]]
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In routed subnets larger than {{IPaddr||31}} or {{IPaddr||32}}, the number of available host addresses is usually reduced by two, namely the largest address, which is reserved as the [[broadcast address]], and the smallest address, which identifies the network itself
In such usage, a {{IPaddr||31}} network, with one binary digit in the host identifier, is unusable, as such a subnet would provide no available host addresses after this reduction. {{IETF RFC|3021}} creates an exception to the "host all ones" and "host all zeros" rules to make {{IPaddr||31}} networks usable for point-to-point links. {{IPaddr||32}} addresses (single-host network) must be accessed by explicit routing rules, as there is no address available for a gateway.
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| colspan="5" | '''T''' = 1,099,511,627,776
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The large address size of IPv6 permitted worldwide route summarization and guaranteed sufficient address pools at each site. The standard subnet size for IPv6 networks is a {{IPaddr||64}} block, which is required for the operation of [[IPv6 stateless address autoconfiguration|stateless address autoconfiguration]].<ref name="RFC 4862">{{IETF RFC|4862}}</ref> At first, the IETF recommended in {{IETF RFC|3177}} as a best practice that all end sites receive
This IPv6 subnetting reference lists the sizes for IPv6 [[subnetwork]]s. Different types of network links may require different subnet sizes.<ref>{{cite web|url=http://www.getipv6.info/index.php/IPv6_Addressing_Plans |title=ARIN IPv6 Addressing Plans |publisher=Getipv6.info |date=2016-03-25 |access-date=2018-03-12}}</ref> The subnet mask separates the bits of the network identifier prefix from the bits of the interface identifier. Selecting a smaller prefix size results in fewer number of networks covered, but with more addresses within each network.<ref>{{cite web |url=https://www.ripe.net/info/info-services/addressing.html |title=RIPE IP Allocation Rates |archive-url=https://web.archive.org/web/20110203130851/http://ripe.net/info/info-services/addressing.html |archive-date=2011-02-03}}</ref>
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|||| |||| |||| |||64 Single LAN; default prefix size for [[IPv6#Stateless_address_autoconfiguration_(SLAAC)|SLAAC]]
|||| |||| |||| ||60 Some (very limited) [[6rd]] deployments (/60 = 16 /64 blocks)
|||| |||| |||| |56 Minimal end
|||| |||| |||| 52 /52 block = 4096 /64 blocks
|||| |||| |||48 Typical assignment for larger sites (/48 = 65536 /64 blocks)
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