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IP subnet calculator

Type an address with its prefix or mask to see the network, its usable range and masks. Split it into smaller subnets, or look up any prefix in the reference tables.

Network21.0.0.0/10
Usable hosts4,194,302
Subnet mask255.192.0.0
Addresses4,194,304
Network address
21.0.0.0
Broadcast address
21.63.255.255
First usable
21.0.0.1
Last usable
21.63.255.254
Usable hosts
4,194,302 (4,194,304 less the network and broadcast addresses)
Subnet mask
255.192.0.0
Wildcard mask
0.63.255.255 (Cisco ACLs, OSPF)
Prefix length
/10
Type
Public public
Old class
A (default /8)
As numbers
Hex 15000000 · Integer 352321536 to 356515839
Reverse DNS zones
0.21.in-addr.arpa
1.21.in-addr.arpa
2.21.in-addr.arpa
3.21.in-addr.arpa
4.21.in-addr.arpa
5.21.in-addr.arpa
6.21.in-addr.arpa
7.21.in-addr.arpa
8.21.in-addr.arpa
9.21.in-addr.arpa
10.21.in-addr.arpa
11.21.in-addr.arpa
12.21.in-addr.arpa
13.21.in-addr.arpa
14.21.in-addr.arpa
15.21.in-addr.arpa
and 48 more
/10 is not on a 8-bit boundary, so it takes 64 /16 zones.
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address00010101.00000000.00000000.0000000021.0.0.0
Subnet mask11111111.11000000.00000000.00000000255.192.0.0
Network00010101.00000000.00000000.0000000021.0.0.0
Broadcast00010101.00111111.11111111.1111111121.63.255.255
Split into smaller subnets/17
/11 (2)/12 (4)/13 (8)/14 (16)/15 (32)/16 (64)/17 (128)/18 (256)/19 (512)/20 (1,024)/21 (2,048)/22 (4,096)

21.0.0.0/10 makes 128 subnets of /17.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
121.0.0.0/1721.0.0.1 to 21.0.127.25421.0.127.25532,766
221.0.128.0/1721.0.128.1 to 21.0.255.25421.0.255.25532,766
321.1.0.0/1721.1.0.1 to 21.1.127.25421.1.127.25532,766
421.1.128.0/1721.1.128.1 to 21.1.255.25421.1.255.25532,766
521.2.0.0/1721.2.0.1 to 21.2.127.25421.2.127.25532,766
621.2.128.0/1721.2.128.1 to 21.2.255.25421.2.255.25532,766
721.3.0.0/1721.3.0.1 to 21.3.127.25421.3.127.25532,766
821.3.128.0/1721.3.128.1 to 21.3.255.25421.3.255.25532,766
921.4.0.0/1721.4.0.1 to 21.4.127.25421.4.127.25532,766
1021.4.128.0/1721.4.128.1 to 21.4.255.25421.4.255.25532,766
1121.5.0.0/1721.5.0.1 to 21.5.127.25421.5.127.25532,766
1221.5.128.0/1721.5.128.1 to 21.5.255.25421.5.255.25532,766
1321.6.0.0/1721.6.0.1 to 21.6.127.25421.6.127.25532,766
1421.6.128.0/1721.6.128.1 to 21.6.255.25421.6.255.25532,766
1521.7.0.0/1721.7.0.1 to 21.7.127.25421.7.127.25532,766
1621.7.128.0/1721.7.128.1 to 21.7.255.25421.7.255.25532,766
1721.8.0.0/1721.8.0.1 to 21.8.127.25421.8.127.25532,766
1821.8.128.0/1721.8.128.1 to 21.8.255.25421.8.255.25532,766
1921.9.0.0/1721.9.0.1 to 21.9.127.25421.9.127.25532,766
2021.9.128.0/1721.9.128.1 to 21.9.255.25421.9.255.25532,766
2121.10.0.0/1721.10.0.1 to 21.10.127.25421.10.127.25532,766
2221.10.128.0/1721.10.128.1 to 21.10.255.25421.10.255.25532,766
2321.11.0.0/1721.11.0.1 to 21.11.127.25421.11.127.25532,766
2421.11.128.0/1721.11.128.1 to 21.11.255.25421.11.255.25532,766
2521.12.0.0/1721.12.0.1 to 21.12.127.25421.12.127.25532,766
2621.12.128.0/1721.12.128.1 to 21.12.255.25421.12.255.25532,766
2721.13.0.0/1721.13.0.1 to 21.13.127.25421.13.127.25532,766
2821.13.128.0/1721.13.128.1 to 21.13.255.25421.13.255.25532,766
2921.14.0.0/1721.14.0.1 to 21.14.127.25421.14.127.25532,766
3021.14.128.0/1721.14.128.1 to 21.14.255.25421.14.255.25532,766
3121.15.0.0/1721.15.0.1 to 21.15.127.25421.15.127.25532,766
3221.15.128.0/1721.15.128.1 to 21.15.255.25421.15.255.25532,766
3321.16.0.0/1721.16.0.1 to 21.16.127.25421.16.127.25532,766
3421.16.128.0/1721.16.128.1 to 21.16.255.25421.16.255.25532,766
3521.17.0.0/1721.17.0.1 to 21.17.127.25421.17.127.25532,766
3621.17.128.0/1721.17.128.1 to 21.17.255.25421.17.255.25532,766
3721.18.0.0/1721.18.0.1 to 21.18.127.25421.18.127.25532,766
3821.18.128.0/1721.18.128.1 to 21.18.255.25421.18.255.25532,766
3921.19.0.0/1721.19.0.1 to 21.19.127.25421.19.127.25532,766
4021.19.128.0/1721.19.128.1 to 21.19.255.25421.19.255.25532,766
4121.20.0.0/1721.20.0.1 to 21.20.127.25421.20.127.25532,766
4221.20.128.0/1721.20.128.1 to 21.20.255.25421.20.255.25532,766
4321.21.0.0/1721.21.0.1 to 21.21.127.25421.21.127.25532,766
4421.21.128.0/1721.21.128.1 to 21.21.255.25421.21.255.25532,766
4521.22.0.0/1721.22.0.1 to 21.22.127.25421.22.127.25532,766
4621.22.128.0/1721.22.128.1 to 21.22.255.25421.22.255.25532,766
4721.23.0.0/1721.23.0.1 to 21.23.127.25421.23.127.25532,766
4821.23.128.0/1721.23.128.1 to 21.23.255.25421.23.255.25532,766
4921.24.0.0/1721.24.0.1 to 21.24.127.25421.24.127.25532,766
5021.24.128.0/1721.24.128.1 to 21.24.255.25421.24.255.25532,766
5121.25.0.0/1721.25.0.1 to 21.25.127.25421.25.127.25532,766
5221.25.128.0/1721.25.128.1 to 21.25.255.25421.25.255.25532,766
5321.26.0.0/1721.26.0.1 to 21.26.127.25421.26.127.25532,766
5421.26.128.0/1721.26.128.1 to 21.26.255.25421.26.255.25532,766
5521.27.0.0/1721.27.0.1 to 21.27.127.25421.27.127.25532,766
5621.27.128.0/1721.27.128.1 to 21.27.255.25421.27.255.25532,766
5721.28.0.0/1721.28.0.1 to 21.28.127.25421.28.127.25532,766
5821.28.128.0/1721.28.128.1 to 21.28.255.25421.28.255.25532,766
5921.29.0.0/1721.29.0.1 to 21.29.127.25421.29.127.25532,766
6021.29.128.0/1721.29.128.1 to 21.29.255.25421.29.255.25532,766
6121.30.0.0/1721.30.0.1 to 21.30.127.25421.30.127.25532,766
6221.30.128.0/1721.30.128.1 to 21.30.255.25421.30.255.25532,766
6321.31.0.0/1721.31.0.1 to 21.31.127.25421.31.127.25532,766
6421.31.128.0/1721.31.128.1 to 21.31.255.25421.31.255.25532,766
6521.32.0.0/1721.32.0.1 to 21.32.127.25421.32.127.25532,766
6621.32.128.0/1721.32.128.1 to 21.32.255.25421.32.255.25532,766
6721.33.0.0/1721.33.0.1 to 21.33.127.25421.33.127.25532,766
6821.33.128.0/1721.33.128.1 to 21.33.255.25421.33.255.25532,766
6921.34.0.0/1721.34.0.1 to 21.34.127.25421.34.127.25532,766
7021.34.128.0/1721.34.128.1 to 21.34.255.25421.34.255.25532,766
7121.35.0.0/1721.35.0.1 to 21.35.127.25421.35.127.25532,766
7221.35.128.0/1721.35.128.1 to 21.35.255.25421.35.255.25532,766
7321.36.0.0/1721.36.0.1 to 21.36.127.25421.36.127.25532,766
7421.36.128.0/1721.36.128.1 to 21.36.255.25421.36.255.25532,766
7521.37.0.0/1721.37.0.1 to 21.37.127.25421.37.127.25532,766
7621.37.128.0/1721.37.128.1 to 21.37.255.25421.37.255.25532,766
7721.38.0.0/1721.38.0.1 to 21.38.127.25421.38.127.25532,766
7821.38.128.0/1721.38.128.1 to 21.38.255.25421.38.255.25532,766
7921.39.0.0/1721.39.0.1 to 21.39.127.25421.39.127.25532,766
8021.39.128.0/1721.39.128.1 to 21.39.255.25421.39.255.25532,766
8121.40.0.0/1721.40.0.1 to 21.40.127.25421.40.127.25532,766
8221.40.128.0/1721.40.128.1 to 21.40.255.25421.40.255.25532,766
8321.41.0.0/1721.41.0.1 to 21.41.127.25421.41.127.25532,766
8421.41.128.0/1721.41.128.1 to 21.41.255.25421.41.255.25532,766
8521.42.0.0/1721.42.0.1 to 21.42.127.25421.42.127.25532,766
8621.42.128.0/1721.42.128.1 to 21.42.255.25421.42.255.25532,766
8721.43.0.0/1721.43.0.1 to 21.43.127.25421.43.127.25532,766
8821.43.128.0/1721.43.128.1 to 21.43.255.25421.43.255.25532,766
8921.44.0.0/1721.44.0.1 to 21.44.127.25421.44.127.25532,766
9021.44.128.0/1721.44.128.1 to 21.44.255.25421.44.255.25532,766
9121.45.0.0/1721.45.0.1 to 21.45.127.25421.45.127.25532,766
9221.45.128.0/1721.45.128.1 to 21.45.255.25421.45.255.25532,766
9321.46.0.0/1721.46.0.1 to 21.46.127.25421.46.127.25532,766
9421.46.128.0/1721.46.128.1 to 21.46.255.25421.46.255.25532,766
9521.47.0.0/1721.47.0.1 to 21.47.127.25421.47.127.25532,766
9621.47.128.0/1721.47.128.1 to 21.47.255.25421.47.255.25532,766
9721.48.0.0/1721.48.0.1 to 21.48.127.25421.48.127.25532,766
9821.48.128.0/1721.48.128.1 to 21.48.255.25421.48.255.25532,766
9921.49.0.0/1721.49.0.1 to 21.49.127.25421.49.127.25532,766
10021.49.128.0/1721.49.128.1 to 21.49.255.25421.49.255.25532,766
10121.50.0.0/1721.50.0.1 to 21.50.127.25421.50.127.25532,766
10221.50.128.0/1721.50.128.1 to 21.50.255.25421.50.255.25532,766
10321.51.0.0/1721.51.0.1 to 21.51.127.25421.51.127.25532,766
10421.51.128.0/1721.51.128.1 to 21.51.255.25421.51.255.25532,766
10521.52.0.0/1721.52.0.1 to 21.52.127.25421.52.127.25532,766
10621.52.128.0/1721.52.128.1 to 21.52.255.25421.52.255.25532,766
10721.53.0.0/1721.53.0.1 to 21.53.127.25421.53.127.25532,766
10821.53.128.0/1721.53.128.1 to 21.53.255.25421.53.255.25532,766
10921.54.0.0/1721.54.0.1 to 21.54.127.25421.54.127.25532,766
11021.54.128.0/1721.54.128.1 to 21.54.255.25421.54.255.25532,766
11121.55.0.0/1721.55.0.1 to 21.55.127.25421.55.127.25532,766
11221.55.128.0/1721.55.128.1 to 21.55.255.25421.55.255.25532,766
11321.56.0.0/1721.56.0.1 to 21.56.127.25421.56.127.25532,766
11421.56.128.0/1721.56.128.1 to 21.56.255.25421.56.255.25532,766
11521.57.0.0/1721.57.0.1 to 21.57.127.25421.57.127.25532,766
11621.57.128.0/1721.57.128.1 to 21.57.255.25421.57.255.25532,766
11721.58.0.0/1721.58.0.1 to 21.58.127.25421.58.127.25532,766
11821.58.128.0/1721.58.128.1 to 21.58.255.25421.58.255.25532,766
11921.59.0.0/1721.59.0.1 to 21.59.127.25421.59.127.25532,766
12021.59.128.0/1721.59.128.1 to 21.59.255.25421.59.255.25532,766
12121.60.0.0/1721.60.0.1 to 21.60.127.25421.60.127.25532,766
12221.60.128.0/1721.60.128.1 to 21.60.255.25421.60.255.25532,766
12321.61.0.0/1721.61.0.1 to 21.61.127.25421.61.127.25532,766
12421.61.128.0/1721.61.128.1 to 21.61.255.25421.61.255.25532,766
12521.62.0.0/1721.62.0.1 to 21.62.127.25421.62.127.25532,766
12621.62.128.0/1721.62.128.1 to 21.62.255.25421.62.255.25532,766
12721.63.0.0/1721.63.0.1 to 21.63.127.25421.63.127.25532,766
12821.63.128.0/1721.63.128.1 to 21.63.255.25421.63.255.25532,766

IPv4 CIDR reference (/0 to /32)

IPv4 prefix lengths
PrefixSubnet maskWildcardAddressesUsable hostsSizeTypical use
/00.0.0.0255.255.255.2554,294,967,2964,294,967,29416,777,216 × /24
/1128.0.0.0127.255.255.2552,147,483,6482,147,483,6468,388,608 × /24
/2192.0.0.063.255.255.2551,073,741,8241,073,741,8224,194,304 × /24
/3224.0.0.031.255.255.255536,870,912536,870,9102,097,152 × /24
/4240.0.0.015.255.255.255268,435,456268,435,4541,048,576 × /24
/5248.0.0.07.255.255.255134,217,728134,217,726524,288 × /24
/6252.0.0.03.255.255.25567,108,86467,108,862262,144 × /24
/7254.0.0.01.255.255.25533,554,43233,554,430131,072 × /24
/8255.0.0.00.255.255.25516,777,21616,777,21465,536 × /24Old class A; 10.0.0.0/8
/9255.128.0.00.127.255.2558,388,6088,388,60632,768 × /24
/10255.192.0.00.63.255.2554,194,3044,194,30216,384 × /24
/11255.224.0.00.31.255.2552,097,1522,097,1508,192 × /24
/12255.240.0.00.15.255.2551,048,5761,048,5744,096 × /24172.16.0.0/12
/13255.248.0.00.7.255.255524,288524,2862,048 × /24
/14255.252.0.00.3.255.255262,144262,1421,024 × /24
/15255.254.0.00.1.255.255131,072131,070512 × /24
/16255.255.0.00.0.255.25565,53665,534256 × /24Old class B; 192.168.0.0/16
/17255.255.128.00.0.127.25532,76832,766128 × /24
/18255.255.192.00.0.63.25516,38416,38264 × /24
/19255.255.224.00.0.31.2558,1928,19032 × /24
/20255.255.240.00.0.15.2554,0964,09416 × /24
/21255.255.248.00.0.7.2552,0482,0468 × /24
/22255.255.252.00.0.3.2551,0241,0224 × /24Large office or guest Wi-Fi (1,022 hosts)
/23255.255.254.00.0.1.2555125102 × /24Larger office LAN (510 hosts)
/24255.255.255.00.0.0.2552562541 × /24Typical office LAN (old class C)
/25255.255.255.1280.0.0.1271281261/2 of a /24Smaller LAN or VLAN
/26255.255.255.1920.0.0.6364621/4 of a /24Smaller LAN or VLAN
/27255.255.255.2240.0.0.3132301/8 of a /24Small VLAN, phones, cameras
/28255.255.255.2400.0.0.1516141/16 of a /24Small VLAN, phones, cameras
/29255.255.255.2480.0.0.7861/32 of a /24Small fixed public block (5 usable)
/30255.255.255.2520.0.0.3421/64 of a /24Point-to-point (old style)
/31255.255.255.2540.0.0.1221/128 of a /24Point-to-point link (RFC 3021)
/32255.255.255.2550.0.0.0111/256 of a /24One host

IPv6 prefix reference

IPv6 prefix lengths
PrefixAddresses/64 LANs/48 sitesNibbleTypical use
/342,535,295,865,117,307,932,921,825,928,971,026,432 (2^125)2,305,843,009,213,693,95235,184,372,088,832NoAll global unicast (2000::/3)
/165,192,296,858,534,827,628,530,496,329,220,096 (2^112)281,474,976,710,6564,294,967,296Yes
/20324,518,553,658,426,726,783,156,020,576,256 (2^108)17,592,186,044,416268,435,456YesLarge provider allocation (RIPE minimum for some)
/29633,825,300,114,114,700,748,351,602,688 (2^99)34,359,738,368524,288NoRIPE allocation to a provider (often)
/3279,228,162,514,264,337,593,543,950,336 (2^96)4,294,967,29665,536YesProvider allocation (RIPE default)
/364,951,760,157,141,521,099,596,496,896 (2^92)268,435,4564,096Yes
/40309,485,009,821,345,068,724,781,056 (2^88)16,777,216256Yes
/4419,342,813,113,834,066,795,298,816 (2^84)1,048,57616Yes
/481,208,925,819,614,629,174,706,176 (2^80)65,5361YesOne site or organisation (RIPE PI / assignment)
/5275,557,863,725,914,323,419,136 (2^76)4,096Yes
/564,722,366,482,869,645,213,696 (2^72)256YesSmall site or home (common provider assignment)
/60295,147,905,179,352,825,856 (2^68)16YesSome home broadband (16 LANs)
/6418,446,744,073,709,551,616 (2^64)1YesOne LAN or VLAN (needed for SLAAC)
/7272,057,594,037,927,936 (2^56)1/256 of a /64Yes
/80281,474,976,710,656 (2^48)1/65,536 of a /64Yes
/964,294,967,2961/4,294,967,296 of a /64YesEmbeds an IPv4 address (NAT64)
/11265,5361/281,474,976,710,656 of a /64Yes
/1202561/72,057,594,037,927,936 of a /64Yes
/124161/1,152,921,504,606,846,976 of a /64Yes
/12641/4,611,686,018,427,387,904 of a /64No
/12721/9,223,372,036,854,775,808 of a /64NoPoint-to-point link (RFC 6164)
/12811/18,446,744,073,709,551,616 of a /64YesOne host or loopback

Private and special ranges

IPv4

Special-purpose ranges
RangeNameWhat it is
255.255.255.255/32Limited broadcastEvery host on the local network
0.0.0.0/8This networkNot routed; 0.0.0.0 means "any address" in settings
10.0.0.0/8Private (RFC 1918)Office and home networks; not routed on the internet
100.64.0.0/10Shared address space (CGNAT)Used by providers behind carrier-grade NAT (RFC 6598)
127.0.0.0/8LoopbackThis computer
169.254.0.0/16Link-local (APIPA)Self-assigned when no DHCP server answers
172.16.0.0/12Private (RFC 1918)Office and home networks; not routed on the internet
192.0.0.0/24IETF protocol assignmentsSpecial uses such as DS-Lite
192.0.2.0/24Documentation (TEST-NET-1)For examples; never used on a network
192.88.99.0/246to4 relay (deprecated)Formerly anycast for 6to4
192.168.0.0/16Private (RFC 1918)Office and home networks; not routed on the internet
198.18.0.0/15BenchmarkingNetwork equipment testing (RFC 2544)
198.51.100.0/24Documentation (TEST-NET-2)For examples; never used on a network
203.0.113.0/24Documentation (TEST-NET-3)For examples; never used on a network
224.0.0.0/4MulticastOne-to-many traffic, such as IPTV and routing protocols
240.0.0.0/4ReservedFuture use; not usable on most systems

IPv6

Special-purpose ranges
RangeNameWhat it is
::/128UnspecifiedNo address; :: means "any address" in settings
::1/128LoopbackThis computer
::ffff:0:0/96IPv4-mappedAn IPv4 address written as IPv6
64:ff9b::/96NAT64IPv4 addresses reached through a NAT64 gateway
100::/64Discard onlyTraffic sent here is dropped (RFC 6666)
2001:db8::/32DocumentationFor examples; never used on a network
2001::/32TeredoIPv6 tunnelled over IPv4 UDP
2002::/166to4IPv6 tunnelled over IPv4 (largely retired)
3fff::/20DocumentationFor examples (RFC 9637)
fc00::/7Unique local (ULA)Private IPv6, like RFC 1918; usually fd00::/8
fe80::/10Link-localEvery IPv6 interface has one; not routed
ff00::/8MulticastOne-to-many traffic; replaces broadcast
2000::/3Global unicastPublic internet addresses

About this tool

A network is written as an address and a prefix length, such as 192.168.1.0/24: the first 24 bits name the network and the rest number the devices on it. The calculator works out where the network starts and ends, how many devices it holds, its subnet and wildcard masks, and the reverse DNS zone to ask your provider for.

It also splits a network into equal smaller ones (one per VLAN or site), turns a range of addresses into the fewest CIDR blocks for a firewall rule, and lists every IPv4 prefix length and the common IPv6 ones in reference tables.

Common questions

Why are two addresses lost in every IPv4 subnet?

The first address names the network and the last is the broadcast address, so a /24 has 256 addresses but 254 for devices. A /31 (a link between two routers) and a /32 (one host) are the exceptions.

What size of IPv6 network should a LAN use?

Always /64. Devices pick their own address within it (SLAAC), which needs the full 64 bits. A site usually gets a /48 or /56 from its provider, which holds 65,536 or 256 LANs.

What is a wildcard mask?

The subnet mask turned inside out: 255.255.255.0 becomes 0.0.0.255. Cisco access lists and OSPF use it to say which bits must match.

Which ranges can I use inside an office?

IPv4: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. IPv6: unique local addresses from fd00::/8, though most networks use the public prefix from their provider.