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Wednesday, November 17, 2010

IPv4 subnetting reference

Source
In the IPv4 address space certain address blocks are specially allocated or reserved for special uses such as loopback interfaces, private networks (RFC 1918), and state-less autoconfiguration (Zeroconf, RFC 3927) of interfaces. Such addresses may be used without registration or allocation from Regional Internet Registries (RIRs). However, these address ranges must not be routed into the public Internet infrastructure.
The netmask is a bitmask that can be used to separate the bits of the network identifier from the bits of the host identifier. It is often written in the same notation used to denote IP addresses.
Not all sizes of prefix announcement may be routable on the public Internet: see routing, peering.
Class Leading
bits
Start End Default
Subnet Mask
in dotted decimal
CIDR notation
A 0 0.0.0.0 127.255.255.255 255.0.0.0 /8
B 10 128.0.0.0 191.255.255.255 255.255.0.0 /16
C 110 192.0.0.0 223.255.255.255 255.255.255.0 /24
D 1110 224.0.0.0 239.255.255.255 not defined not defined
E 1111 240.0.0.0 255.255.255.254 not defined not defined
The blocks numerically at the start and end of classes A, B and C were originally reserved for special addressing or future features, i.e., 0.0.0.0/8 and 127.0.0.0/8 are reserved in former class A; 128.0.0.0/16 and 191.255.0.0/16 are reserved in former class B; 192.0.0.0/24 and 223.255.255.0/24 are reserved in former class C.
While the 127.0.0.0/8 network is a Class A network, it is designated for loopback and cannot be assigned to a network.
CIDR host
bits
Netmask Hosts in
subnet
Classful name Typical usage
/8 24 255.0.0.0 16777216 = 224 Class A
(see this list)
Largest block
allocation
made by IANA
/9 23 255.128.0.0 8388608 = 223

/10 22 255.192.0.0 4194304 = 222

/11 21 255.224.0.0 2097152 = 221

/12 20 255.240.0.0 1048576 = 220

/13 19 255.248.0.0 524288 = 219

/14 18 255.252.0.0 262144 = 218

/15 17 255.254.0.0 131072 = 217

/16 16 255.255.0.0 65536 = 216 Class B
/17 15 255.255.128.0 32768 = 215
ISP /
large business
/18 14 255.255.192.0 16384 = 214
ISP /
large business
/19 13 255.255.224.0 8192 = 213
ISP /
large business
/20 12 255.255.240.0 4096 = 212
Small ISP /
large business
/21 11 255.255.248.0 2048 = 211
Small ISP /
large business
/22 10 255.255.252.0 1024 = 210

/23 9 255.255.254.0 512 = 29

/24 8 255.255.255.0 256 = 28 Class C Large LAN
/25 7 255.255.255.128 128 = 27
Large LAN
/26 6 255.255.255.192 64 = 26
Small LAN
/27 5 255.255.255.224 32 = 25
Small LAN
/28 4 255.255.255.240 16 = 24
Small LAN
/29 3 255.255.255.248 8 = 23
Smallest
multi-host
network
/30 2 255.255.255.252 4 = 22
"Glue network"
(point to point
links)
/31 1 255.255.255.254 2 = 21
Rarely used,
point to point
links (RFC 3021)
/32 0 255.255.255.255 1 = 20
Host route
In common usage, the "host all zeros" address is reserved for referring to the entire network, while the "host all ones" address is used as a broadcast address in the given subnet; this reduces the number of addresses available for hosts by 2. This explains the reference to /31 networks as "Rarely Used," as the only possible addresses on a /31 network are "host all ones" and "host all zeros." RFC 3021 creates an exception to the "host all ones" and "host all zeros" broadcast usage to make /31 networks usable for point-to-point links. In practice, however, point-to-point links are still typically implemented using /30 networks, or occasionally by /32 and point-to-point explicit host routes.[citation needed] There is generally no technical advantage to /31 versus /32, although one or the other may be more convenient based on other issues. A /30 is always wasteful and has as its sole advantage that it behaves "as expected" for any other subnetwork.

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