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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.

Network21.0.0.0/8
Usable hosts16,777,214
Subnet mask255.0.0.0
Addresses16,777,216
Network address
21.0.0.0
Broadcast address
21.255.255.255
First usable
21.0.0.1
Last usable
21.255.255.254
Usable hosts
16,777,214 (16,777,216 less the network and broadcast addresses)
Subnet mask
255.0.0.0
Wildcard mask
0.255.255.255 (Cisco ACLs, OSPF)
Prefix length
/8
Type
Public public
Old class
A (default /8)
As numbers
Hex 15000000 · Integer 352321536 to 369098751
Reverse DNS zone
21.in-addr.arpa
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address00010101..00000000.00000000.0000000021.0.0.0
Subnet mask11111111..00000000.00000000.00000000255.0.0.0
Network00010101..00000000.00000000.0000000021.0.0.0
Broadcast00010101..11111111.11111111.1111111121.255.255.255
Split into smaller subnets/15
/9 (2)/10 (4)/11 (8)/12 (16)/13 (32)/14 (64)/15 (128)/16 (256)/17 (512)/18 (1,024)/19 (2,048)/20 (4,096)

21.0.0.0/8 makes 128 subnets of /15.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
121.0.0.0/1521.0.0.1 to 21.1.255.25421.1.255.255131,070
221.2.0.0/1521.2.0.1 to 21.3.255.25421.3.255.255131,070
321.4.0.0/1521.4.0.1 to 21.5.255.25421.5.255.255131,070
421.6.0.0/1521.6.0.1 to 21.7.255.25421.7.255.255131,070
521.8.0.0/1521.8.0.1 to 21.9.255.25421.9.255.255131,070
621.10.0.0/1521.10.0.1 to 21.11.255.25421.11.255.255131,070
721.12.0.0/1521.12.0.1 to 21.13.255.25421.13.255.255131,070
821.14.0.0/1521.14.0.1 to 21.15.255.25421.15.255.255131,070
921.16.0.0/1521.16.0.1 to 21.17.255.25421.17.255.255131,070
1021.18.0.0/1521.18.0.1 to 21.19.255.25421.19.255.255131,070
1121.20.0.0/1521.20.0.1 to 21.21.255.25421.21.255.255131,070
1221.22.0.0/1521.22.0.1 to 21.23.255.25421.23.255.255131,070
1321.24.0.0/1521.24.0.1 to 21.25.255.25421.25.255.255131,070
1421.26.0.0/1521.26.0.1 to 21.27.255.25421.27.255.255131,070
1521.28.0.0/1521.28.0.1 to 21.29.255.25421.29.255.255131,070
1621.30.0.0/1521.30.0.1 to 21.31.255.25421.31.255.255131,070
1721.32.0.0/1521.32.0.1 to 21.33.255.25421.33.255.255131,070
1821.34.0.0/1521.34.0.1 to 21.35.255.25421.35.255.255131,070
1921.36.0.0/1521.36.0.1 to 21.37.255.25421.37.255.255131,070
2021.38.0.0/1521.38.0.1 to 21.39.255.25421.39.255.255131,070
2121.40.0.0/1521.40.0.1 to 21.41.255.25421.41.255.255131,070
2221.42.0.0/1521.42.0.1 to 21.43.255.25421.43.255.255131,070
2321.44.0.0/1521.44.0.1 to 21.45.255.25421.45.255.255131,070
2421.46.0.0/1521.46.0.1 to 21.47.255.25421.47.255.255131,070
2521.48.0.0/1521.48.0.1 to 21.49.255.25421.49.255.255131,070
2621.50.0.0/1521.50.0.1 to 21.51.255.25421.51.255.255131,070
2721.52.0.0/1521.52.0.1 to 21.53.255.25421.53.255.255131,070
2821.54.0.0/1521.54.0.1 to 21.55.255.25421.55.255.255131,070
2921.56.0.0/1521.56.0.1 to 21.57.255.25421.57.255.255131,070
3021.58.0.0/1521.58.0.1 to 21.59.255.25421.59.255.255131,070
3121.60.0.0/1521.60.0.1 to 21.61.255.25421.61.255.255131,070
3221.62.0.0/1521.62.0.1 to 21.63.255.25421.63.255.255131,070
3321.64.0.0/1521.64.0.1 to 21.65.255.25421.65.255.255131,070
3421.66.0.0/1521.66.0.1 to 21.67.255.25421.67.255.255131,070
3521.68.0.0/1521.68.0.1 to 21.69.255.25421.69.255.255131,070
3621.70.0.0/1521.70.0.1 to 21.71.255.25421.71.255.255131,070
3721.72.0.0/1521.72.0.1 to 21.73.255.25421.73.255.255131,070
3821.74.0.0/1521.74.0.1 to 21.75.255.25421.75.255.255131,070
3921.76.0.0/1521.76.0.1 to 21.77.255.25421.77.255.255131,070
4021.78.0.0/1521.78.0.1 to 21.79.255.25421.79.255.255131,070
4121.80.0.0/1521.80.0.1 to 21.81.255.25421.81.255.255131,070
4221.82.0.0/1521.82.0.1 to 21.83.255.25421.83.255.255131,070
4321.84.0.0/1521.84.0.1 to 21.85.255.25421.85.255.255131,070
4421.86.0.0/1521.86.0.1 to 21.87.255.25421.87.255.255131,070
4521.88.0.0/1521.88.0.1 to 21.89.255.25421.89.255.255131,070
4621.90.0.0/1521.90.0.1 to 21.91.255.25421.91.255.255131,070
4721.92.0.0/1521.92.0.1 to 21.93.255.25421.93.255.255131,070
4821.94.0.0/1521.94.0.1 to 21.95.255.25421.95.255.255131,070
4921.96.0.0/1521.96.0.1 to 21.97.255.25421.97.255.255131,070
5021.98.0.0/1521.98.0.1 to 21.99.255.25421.99.255.255131,070
5121.100.0.0/1521.100.0.1 to 21.101.255.25421.101.255.255131,070
5221.102.0.0/1521.102.0.1 to 21.103.255.25421.103.255.255131,070
5321.104.0.0/1521.104.0.1 to 21.105.255.25421.105.255.255131,070
5421.106.0.0/1521.106.0.1 to 21.107.255.25421.107.255.255131,070
5521.108.0.0/1521.108.0.1 to 21.109.255.25421.109.255.255131,070
5621.110.0.0/1521.110.0.1 to 21.111.255.25421.111.255.255131,070
5721.112.0.0/1521.112.0.1 to 21.113.255.25421.113.255.255131,070
5821.114.0.0/1521.114.0.1 to 21.115.255.25421.115.255.255131,070
5921.116.0.0/1521.116.0.1 to 21.117.255.25421.117.255.255131,070
6021.118.0.0/1521.118.0.1 to 21.119.255.25421.119.255.255131,070
6121.120.0.0/1521.120.0.1 to 21.121.255.25421.121.255.255131,070
6221.122.0.0/1521.122.0.1 to 21.123.255.25421.123.255.255131,070
6321.124.0.0/1521.124.0.1 to 21.125.255.25421.125.255.255131,070
6421.126.0.0/1521.126.0.1 to 21.127.255.25421.127.255.255131,070
6521.128.0.0/1521.128.0.1 to 21.129.255.25421.129.255.255131,070
6621.130.0.0/1521.130.0.1 to 21.131.255.25421.131.255.255131,070
6721.132.0.0/1521.132.0.1 to 21.133.255.25421.133.255.255131,070
6821.134.0.0/1521.134.0.1 to 21.135.255.25421.135.255.255131,070
6921.136.0.0/1521.136.0.1 to 21.137.255.25421.137.255.255131,070
7021.138.0.0/1521.138.0.1 to 21.139.255.25421.139.255.255131,070
7121.140.0.0/1521.140.0.1 to 21.141.255.25421.141.255.255131,070
7221.142.0.0/1521.142.0.1 to 21.143.255.25421.143.255.255131,070
7321.144.0.0/1521.144.0.1 to 21.145.255.25421.145.255.255131,070
7421.146.0.0/1521.146.0.1 to 21.147.255.25421.147.255.255131,070
7521.148.0.0/1521.148.0.1 to 21.149.255.25421.149.255.255131,070
7621.150.0.0/1521.150.0.1 to 21.151.255.25421.151.255.255131,070
7721.152.0.0/1521.152.0.1 to 21.153.255.25421.153.255.255131,070
7821.154.0.0/1521.154.0.1 to 21.155.255.25421.155.255.255131,070
7921.156.0.0/1521.156.0.1 to 21.157.255.25421.157.255.255131,070
8021.158.0.0/1521.158.0.1 to 21.159.255.25421.159.255.255131,070
8121.160.0.0/1521.160.0.1 to 21.161.255.25421.161.255.255131,070
8221.162.0.0/1521.162.0.1 to 21.163.255.25421.163.255.255131,070
8321.164.0.0/1521.164.0.1 to 21.165.255.25421.165.255.255131,070
8421.166.0.0/1521.166.0.1 to 21.167.255.25421.167.255.255131,070
8521.168.0.0/1521.168.0.1 to 21.169.255.25421.169.255.255131,070
8621.170.0.0/1521.170.0.1 to 21.171.255.25421.171.255.255131,070
8721.172.0.0/1521.172.0.1 to 21.173.255.25421.173.255.255131,070
8821.174.0.0/1521.174.0.1 to 21.175.255.25421.175.255.255131,070
8921.176.0.0/1521.176.0.1 to 21.177.255.25421.177.255.255131,070
9021.178.0.0/1521.178.0.1 to 21.179.255.25421.179.255.255131,070
9121.180.0.0/1521.180.0.1 to 21.181.255.25421.181.255.255131,070
9221.182.0.0/1521.182.0.1 to 21.183.255.25421.183.255.255131,070
9321.184.0.0/1521.184.0.1 to 21.185.255.25421.185.255.255131,070
9421.186.0.0/1521.186.0.1 to 21.187.255.25421.187.255.255131,070
9521.188.0.0/1521.188.0.1 to 21.189.255.25421.189.255.255131,070
9621.190.0.0/1521.190.0.1 to 21.191.255.25421.191.255.255131,070
9721.192.0.0/1521.192.0.1 to 21.193.255.25421.193.255.255131,070
9821.194.0.0/1521.194.0.1 to 21.195.255.25421.195.255.255131,070
9921.196.0.0/1521.196.0.1 to 21.197.255.25421.197.255.255131,070
10021.198.0.0/1521.198.0.1 to 21.199.255.25421.199.255.255131,070
10121.200.0.0/1521.200.0.1 to 21.201.255.25421.201.255.255131,070
10221.202.0.0/1521.202.0.1 to 21.203.255.25421.203.255.255131,070
10321.204.0.0/1521.204.0.1 to 21.205.255.25421.205.255.255131,070
10421.206.0.0/1521.206.0.1 to 21.207.255.25421.207.255.255131,070
10521.208.0.0/1521.208.0.1 to 21.209.255.25421.209.255.255131,070
10621.210.0.0/1521.210.0.1 to 21.211.255.25421.211.255.255131,070
10721.212.0.0/1521.212.0.1 to 21.213.255.25421.213.255.255131,070
10821.214.0.0/1521.214.0.1 to 21.215.255.25421.215.255.255131,070
10921.216.0.0/1521.216.0.1 to 21.217.255.25421.217.255.255131,070
11021.218.0.0/1521.218.0.1 to 21.219.255.25421.219.255.255131,070
11121.220.0.0/1521.220.0.1 to 21.221.255.25421.221.255.255131,070
11221.222.0.0/1521.222.0.1 to 21.223.255.25421.223.255.255131,070
11321.224.0.0/1521.224.0.1 to 21.225.255.25421.225.255.255131,070
11421.226.0.0/1521.226.0.1 to 21.227.255.25421.227.255.255131,070
11521.228.0.0/1521.228.0.1 to 21.229.255.25421.229.255.255131,070
11621.230.0.0/1521.230.0.1 to 21.231.255.25421.231.255.255131,070
11721.232.0.0/1521.232.0.1 to 21.233.255.25421.233.255.255131,070
11821.234.0.0/1521.234.0.1 to 21.235.255.25421.235.255.255131,070
11921.236.0.0/1521.236.0.1 to 21.237.255.25421.237.255.255131,070
12021.238.0.0/1521.238.0.1 to 21.239.255.25421.239.255.255131,070
12121.240.0.0/1521.240.0.1 to 21.241.255.25421.241.255.255131,070
12221.242.0.0/1521.242.0.1 to 21.243.255.25421.243.255.255131,070
12321.244.0.0/1521.244.0.1 to 21.245.255.25421.245.255.255131,070
12421.246.0.0/1521.246.0.1 to 21.247.255.25421.247.255.255131,070
12521.248.0.0/1521.248.0.1 to 21.249.255.25421.249.255.255131,070
12621.250.0.0/1521.250.0.1 to 21.251.255.25421.251.255.255131,070
12721.252.0.0/1521.252.0.1 to 21.253.255.25421.253.255.255131,070
12821.254.0.0/1521.254.0.1 to 21.255.255.25421.255.255.255131,070

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.