Speed and call quality testMy IP addressIP subnet calculatorDNS reportWhoIs lookupRemote support

Internet and network

Speed and call quality testTest delay, jitter and speed, with a focus on phone calls.My IP addressYour public IPv4 and IPv6 addresses, provider and address range.WhoIs lookupWho owns a domain or IP address, when it expires, and its DNS.DNS reportEvery DNS record for a domain, with email setup checked.IP subnet calculatorWork out a network's range, mask and hosts, and split it into subnets.

Computers and devices

Update checkIs this computer, phone and browser up to date? With links to update.Camera and mic testCheck your webcam and microphone before a call.Test print pagePrint an A4 page to check a printer: edges, text, line art and colours.

Make and print

Clean textStrip hidden characters, smart quotes and odd spaces from pasted text.QR code makerMake a QR code for a link, contact or message. Nothing leaves your browser.Wi-Fi QR posterPrint an A4 poster that lets guests join your Wi-Fi by scanning.

Surftec support

Remote supportLet a Surftec engineer connect to your computer to help you.All workingA full-screen confirmation that everything is up and running.

Surftec

Surftec websiteServices, news and how to contact Surftec
Cloud Tools
Guest
GuestNot signed in

Surftec customers will be recognised automatically when they use these tools from their office network.

Customer sign-in is coming soon.

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.

Network21.0.0.0/9
Usable hosts8,388,606
Subnet mask255.128.0.0
Addresses8,388,608
Network address
21.0.0.0
Broadcast address
21.127.255.255
First usable
21.0.0.1
Last usable
21.127.255.254
Usable hosts
8,388,606 (8,388,608 less the network and broadcast addresses)
Subnet mask
255.128.0.0
Wildcard mask
0.127.255.255 (Cisco ACLs, OSPF)
Prefix length
/9
Type
Public public
Old class
A (default /8)
As numbers
Hex 15000000 · Integer 352321536 to 360710143
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 112 more
/9 is not on a 8-bit boundary, so it takes 128 /16 zones.
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address00010101.00000000.00000000.0000000021.0.0.0
Subnet mask11111111.10000000.00000000.00000000255.128.0.0
Network00010101.00000000.00000000.0000000021.0.0.0
Broadcast00010101.01111111.11111111.1111111121.127.255.255
Split into smaller subnets/18
/10 (2)/11 (4)/12 (8)/13 (16)/14 (32)/15 (64)/16 (128)/17 (256)/18 (512)/19 (1,024)/20 (2,048)/21 (4,096)

21.0.0.0/9 makes 512 subnets of /18; the first 256 are listed.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
121.0.0.0/1821.0.0.1 to 21.0.63.25421.0.63.25516,382
221.0.64.0/1821.0.64.1 to 21.0.127.25421.0.127.25516,382
321.0.128.0/1821.0.128.1 to 21.0.191.25421.0.191.25516,382
421.0.192.0/1821.0.192.1 to 21.0.255.25421.0.255.25516,382
521.1.0.0/1821.1.0.1 to 21.1.63.25421.1.63.25516,382
621.1.64.0/1821.1.64.1 to 21.1.127.25421.1.127.25516,382
721.1.128.0/1821.1.128.1 to 21.1.191.25421.1.191.25516,382
821.1.192.0/1821.1.192.1 to 21.1.255.25421.1.255.25516,382
921.2.0.0/1821.2.0.1 to 21.2.63.25421.2.63.25516,382
1021.2.64.0/1821.2.64.1 to 21.2.127.25421.2.127.25516,382
1121.2.128.0/1821.2.128.1 to 21.2.191.25421.2.191.25516,382
1221.2.192.0/1821.2.192.1 to 21.2.255.25421.2.255.25516,382
1321.3.0.0/1821.3.0.1 to 21.3.63.25421.3.63.25516,382
1421.3.64.0/1821.3.64.1 to 21.3.127.25421.3.127.25516,382
1521.3.128.0/1821.3.128.1 to 21.3.191.25421.3.191.25516,382
1621.3.192.0/1821.3.192.1 to 21.3.255.25421.3.255.25516,382
1721.4.0.0/1821.4.0.1 to 21.4.63.25421.4.63.25516,382
1821.4.64.0/1821.4.64.1 to 21.4.127.25421.4.127.25516,382
1921.4.128.0/1821.4.128.1 to 21.4.191.25421.4.191.25516,382
2021.4.192.0/1821.4.192.1 to 21.4.255.25421.4.255.25516,382
2121.5.0.0/1821.5.0.1 to 21.5.63.25421.5.63.25516,382
2221.5.64.0/1821.5.64.1 to 21.5.127.25421.5.127.25516,382
2321.5.128.0/1821.5.128.1 to 21.5.191.25421.5.191.25516,382
2421.5.192.0/1821.5.192.1 to 21.5.255.25421.5.255.25516,382
2521.6.0.0/1821.6.0.1 to 21.6.63.25421.6.63.25516,382
2621.6.64.0/1821.6.64.1 to 21.6.127.25421.6.127.25516,382
2721.6.128.0/1821.6.128.1 to 21.6.191.25421.6.191.25516,382
2821.6.192.0/1821.6.192.1 to 21.6.255.25421.6.255.25516,382
2921.7.0.0/1821.7.0.1 to 21.7.63.25421.7.63.25516,382
3021.7.64.0/1821.7.64.1 to 21.7.127.25421.7.127.25516,382
3121.7.128.0/1821.7.128.1 to 21.7.191.25421.7.191.25516,382
3221.7.192.0/1821.7.192.1 to 21.7.255.25421.7.255.25516,382
3321.8.0.0/1821.8.0.1 to 21.8.63.25421.8.63.25516,382
3421.8.64.0/1821.8.64.1 to 21.8.127.25421.8.127.25516,382
3521.8.128.0/1821.8.128.1 to 21.8.191.25421.8.191.25516,382
3621.8.192.0/1821.8.192.1 to 21.8.255.25421.8.255.25516,382
3721.9.0.0/1821.9.0.1 to 21.9.63.25421.9.63.25516,382
3821.9.64.0/1821.9.64.1 to 21.9.127.25421.9.127.25516,382
3921.9.128.0/1821.9.128.1 to 21.9.191.25421.9.191.25516,382
4021.9.192.0/1821.9.192.1 to 21.9.255.25421.9.255.25516,382
4121.10.0.0/1821.10.0.1 to 21.10.63.25421.10.63.25516,382
4221.10.64.0/1821.10.64.1 to 21.10.127.25421.10.127.25516,382
4321.10.128.0/1821.10.128.1 to 21.10.191.25421.10.191.25516,382
4421.10.192.0/1821.10.192.1 to 21.10.255.25421.10.255.25516,382
4521.11.0.0/1821.11.0.1 to 21.11.63.25421.11.63.25516,382
4621.11.64.0/1821.11.64.1 to 21.11.127.25421.11.127.25516,382
4721.11.128.0/1821.11.128.1 to 21.11.191.25421.11.191.25516,382
4821.11.192.0/1821.11.192.1 to 21.11.255.25421.11.255.25516,382
4921.12.0.0/1821.12.0.1 to 21.12.63.25421.12.63.25516,382
5021.12.64.0/1821.12.64.1 to 21.12.127.25421.12.127.25516,382
5121.12.128.0/1821.12.128.1 to 21.12.191.25421.12.191.25516,382
5221.12.192.0/1821.12.192.1 to 21.12.255.25421.12.255.25516,382
5321.13.0.0/1821.13.0.1 to 21.13.63.25421.13.63.25516,382
5421.13.64.0/1821.13.64.1 to 21.13.127.25421.13.127.25516,382
5521.13.128.0/1821.13.128.1 to 21.13.191.25421.13.191.25516,382
5621.13.192.0/1821.13.192.1 to 21.13.255.25421.13.255.25516,382
5721.14.0.0/1821.14.0.1 to 21.14.63.25421.14.63.25516,382
5821.14.64.0/1821.14.64.1 to 21.14.127.25421.14.127.25516,382
5921.14.128.0/1821.14.128.1 to 21.14.191.25421.14.191.25516,382
6021.14.192.0/1821.14.192.1 to 21.14.255.25421.14.255.25516,382
6121.15.0.0/1821.15.0.1 to 21.15.63.25421.15.63.25516,382
6221.15.64.0/1821.15.64.1 to 21.15.127.25421.15.127.25516,382
6321.15.128.0/1821.15.128.1 to 21.15.191.25421.15.191.25516,382
6421.15.192.0/1821.15.192.1 to 21.15.255.25421.15.255.25516,382
6521.16.0.0/1821.16.0.1 to 21.16.63.25421.16.63.25516,382
6621.16.64.0/1821.16.64.1 to 21.16.127.25421.16.127.25516,382
6721.16.128.0/1821.16.128.1 to 21.16.191.25421.16.191.25516,382
6821.16.192.0/1821.16.192.1 to 21.16.255.25421.16.255.25516,382
6921.17.0.0/1821.17.0.1 to 21.17.63.25421.17.63.25516,382
7021.17.64.0/1821.17.64.1 to 21.17.127.25421.17.127.25516,382
7121.17.128.0/1821.17.128.1 to 21.17.191.25421.17.191.25516,382
7221.17.192.0/1821.17.192.1 to 21.17.255.25421.17.255.25516,382
7321.18.0.0/1821.18.0.1 to 21.18.63.25421.18.63.25516,382
7421.18.64.0/1821.18.64.1 to 21.18.127.25421.18.127.25516,382
7521.18.128.0/1821.18.128.1 to 21.18.191.25421.18.191.25516,382
7621.18.192.0/1821.18.192.1 to 21.18.255.25421.18.255.25516,382
7721.19.0.0/1821.19.0.1 to 21.19.63.25421.19.63.25516,382
7821.19.64.0/1821.19.64.1 to 21.19.127.25421.19.127.25516,382
7921.19.128.0/1821.19.128.1 to 21.19.191.25421.19.191.25516,382
8021.19.192.0/1821.19.192.1 to 21.19.255.25421.19.255.25516,382
8121.20.0.0/1821.20.0.1 to 21.20.63.25421.20.63.25516,382
8221.20.64.0/1821.20.64.1 to 21.20.127.25421.20.127.25516,382
8321.20.128.0/1821.20.128.1 to 21.20.191.25421.20.191.25516,382
8421.20.192.0/1821.20.192.1 to 21.20.255.25421.20.255.25516,382
8521.21.0.0/1821.21.0.1 to 21.21.63.25421.21.63.25516,382
8621.21.64.0/1821.21.64.1 to 21.21.127.25421.21.127.25516,382
8721.21.128.0/1821.21.128.1 to 21.21.191.25421.21.191.25516,382
8821.21.192.0/1821.21.192.1 to 21.21.255.25421.21.255.25516,382
8921.22.0.0/1821.22.0.1 to 21.22.63.25421.22.63.25516,382
9021.22.64.0/1821.22.64.1 to 21.22.127.25421.22.127.25516,382
9121.22.128.0/1821.22.128.1 to 21.22.191.25421.22.191.25516,382
9221.22.192.0/1821.22.192.1 to 21.22.255.25421.22.255.25516,382
9321.23.0.0/1821.23.0.1 to 21.23.63.25421.23.63.25516,382
9421.23.64.0/1821.23.64.1 to 21.23.127.25421.23.127.25516,382
9521.23.128.0/1821.23.128.1 to 21.23.191.25421.23.191.25516,382
9621.23.192.0/1821.23.192.1 to 21.23.255.25421.23.255.25516,382
9721.24.0.0/1821.24.0.1 to 21.24.63.25421.24.63.25516,382
9821.24.64.0/1821.24.64.1 to 21.24.127.25421.24.127.25516,382
9921.24.128.0/1821.24.128.1 to 21.24.191.25421.24.191.25516,382
10021.24.192.0/1821.24.192.1 to 21.24.255.25421.24.255.25516,382
10121.25.0.0/1821.25.0.1 to 21.25.63.25421.25.63.25516,382
10221.25.64.0/1821.25.64.1 to 21.25.127.25421.25.127.25516,382
10321.25.128.0/1821.25.128.1 to 21.25.191.25421.25.191.25516,382
10421.25.192.0/1821.25.192.1 to 21.25.255.25421.25.255.25516,382
10521.26.0.0/1821.26.0.1 to 21.26.63.25421.26.63.25516,382
10621.26.64.0/1821.26.64.1 to 21.26.127.25421.26.127.25516,382
10721.26.128.0/1821.26.128.1 to 21.26.191.25421.26.191.25516,382
10821.26.192.0/1821.26.192.1 to 21.26.255.25421.26.255.25516,382
10921.27.0.0/1821.27.0.1 to 21.27.63.25421.27.63.25516,382
11021.27.64.0/1821.27.64.1 to 21.27.127.25421.27.127.25516,382
11121.27.128.0/1821.27.128.1 to 21.27.191.25421.27.191.25516,382
11221.27.192.0/1821.27.192.1 to 21.27.255.25421.27.255.25516,382
11321.28.0.0/1821.28.0.1 to 21.28.63.25421.28.63.25516,382
11421.28.64.0/1821.28.64.1 to 21.28.127.25421.28.127.25516,382
11521.28.128.0/1821.28.128.1 to 21.28.191.25421.28.191.25516,382
11621.28.192.0/1821.28.192.1 to 21.28.255.25421.28.255.25516,382
11721.29.0.0/1821.29.0.1 to 21.29.63.25421.29.63.25516,382
11821.29.64.0/1821.29.64.1 to 21.29.127.25421.29.127.25516,382
11921.29.128.0/1821.29.128.1 to 21.29.191.25421.29.191.25516,382
12021.29.192.0/1821.29.192.1 to 21.29.255.25421.29.255.25516,382
12121.30.0.0/1821.30.0.1 to 21.30.63.25421.30.63.25516,382
12221.30.64.0/1821.30.64.1 to 21.30.127.25421.30.127.25516,382
12321.30.128.0/1821.30.128.1 to 21.30.191.25421.30.191.25516,382
12421.30.192.0/1821.30.192.1 to 21.30.255.25421.30.255.25516,382
12521.31.0.0/1821.31.0.1 to 21.31.63.25421.31.63.25516,382
12621.31.64.0/1821.31.64.1 to 21.31.127.25421.31.127.25516,382
12721.31.128.0/1821.31.128.1 to 21.31.191.25421.31.191.25516,382
12821.31.192.0/1821.31.192.1 to 21.31.255.25421.31.255.25516,382
12921.32.0.0/1821.32.0.1 to 21.32.63.25421.32.63.25516,382
13021.32.64.0/1821.32.64.1 to 21.32.127.25421.32.127.25516,382
13121.32.128.0/1821.32.128.1 to 21.32.191.25421.32.191.25516,382
13221.32.192.0/1821.32.192.1 to 21.32.255.25421.32.255.25516,382
13321.33.0.0/1821.33.0.1 to 21.33.63.25421.33.63.25516,382
13421.33.64.0/1821.33.64.1 to 21.33.127.25421.33.127.25516,382
13521.33.128.0/1821.33.128.1 to 21.33.191.25421.33.191.25516,382
13621.33.192.0/1821.33.192.1 to 21.33.255.25421.33.255.25516,382
13721.34.0.0/1821.34.0.1 to 21.34.63.25421.34.63.25516,382
13821.34.64.0/1821.34.64.1 to 21.34.127.25421.34.127.25516,382
13921.34.128.0/1821.34.128.1 to 21.34.191.25421.34.191.25516,382
14021.34.192.0/1821.34.192.1 to 21.34.255.25421.34.255.25516,382
14121.35.0.0/1821.35.0.1 to 21.35.63.25421.35.63.25516,382
14221.35.64.0/1821.35.64.1 to 21.35.127.25421.35.127.25516,382
14321.35.128.0/1821.35.128.1 to 21.35.191.25421.35.191.25516,382
14421.35.192.0/1821.35.192.1 to 21.35.255.25421.35.255.25516,382
14521.36.0.0/1821.36.0.1 to 21.36.63.25421.36.63.25516,382
14621.36.64.0/1821.36.64.1 to 21.36.127.25421.36.127.25516,382
14721.36.128.0/1821.36.128.1 to 21.36.191.25421.36.191.25516,382
14821.36.192.0/1821.36.192.1 to 21.36.255.25421.36.255.25516,382
14921.37.0.0/1821.37.0.1 to 21.37.63.25421.37.63.25516,382
15021.37.64.0/1821.37.64.1 to 21.37.127.25421.37.127.25516,382
15121.37.128.0/1821.37.128.1 to 21.37.191.25421.37.191.25516,382
15221.37.192.0/1821.37.192.1 to 21.37.255.25421.37.255.25516,382
15321.38.0.0/1821.38.0.1 to 21.38.63.25421.38.63.25516,382
15421.38.64.0/1821.38.64.1 to 21.38.127.25421.38.127.25516,382
15521.38.128.0/1821.38.128.1 to 21.38.191.25421.38.191.25516,382
15621.38.192.0/1821.38.192.1 to 21.38.255.25421.38.255.25516,382
15721.39.0.0/1821.39.0.1 to 21.39.63.25421.39.63.25516,382
15821.39.64.0/1821.39.64.1 to 21.39.127.25421.39.127.25516,382
15921.39.128.0/1821.39.128.1 to 21.39.191.25421.39.191.25516,382
16021.39.192.0/1821.39.192.1 to 21.39.255.25421.39.255.25516,382
16121.40.0.0/1821.40.0.1 to 21.40.63.25421.40.63.25516,382
16221.40.64.0/1821.40.64.1 to 21.40.127.25421.40.127.25516,382
16321.40.128.0/1821.40.128.1 to 21.40.191.25421.40.191.25516,382
16421.40.192.0/1821.40.192.1 to 21.40.255.25421.40.255.25516,382
16521.41.0.0/1821.41.0.1 to 21.41.63.25421.41.63.25516,382
16621.41.64.0/1821.41.64.1 to 21.41.127.25421.41.127.25516,382
16721.41.128.0/1821.41.128.1 to 21.41.191.25421.41.191.25516,382
16821.41.192.0/1821.41.192.1 to 21.41.255.25421.41.255.25516,382
16921.42.0.0/1821.42.0.1 to 21.42.63.25421.42.63.25516,382
17021.42.64.0/1821.42.64.1 to 21.42.127.25421.42.127.25516,382
17121.42.128.0/1821.42.128.1 to 21.42.191.25421.42.191.25516,382
17221.42.192.0/1821.42.192.1 to 21.42.255.25421.42.255.25516,382
17321.43.0.0/1821.43.0.1 to 21.43.63.25421.43.63.25516,382
17421.43.64.0/1821.43.64.1 to 21.43.127.25421.43.127.25516,382
17521.43.128.0/1821.43.128.1 to 21.43.191.25421.43.191.25516,382
17621.43.192.0/1821.43.192.1 to 21.43.255.25421.43.255.25516,382
17721.44.0.0/1821.44.0.1 to 21.44.63.25421.44.63.25516,382
17821.44.64.0/1821.44.64.1 to 21.44.127.25421.44.127.25516,382
17921.44.128.0/1821.44.128.1 to 21.44.191.25421.44.191.25516,382
18021.44.192.0/1821.44.192.1 to 21.44.255.25421.44.255.25516,382
18121.45.0.0/1821.45.0.1 to 21.45.63.25421.45.63.25516,382
18221.45.64.0/1821.45.64.1 to 21.45.127.25421.45.127.25516,382
18321.45.128.0/1821.45.128.1 to 21.45.191.25421.45.191.25516,382
18421.45.192.0/1821.45.192.1 to 21.45.255.25421.45.255.25516,382
18521.46.0.0/1821.46.0.1 to 21.46.63.25421.46.63.25516,382
18621.46.64.0/1821.46.64.1 to 21.46.127.25421.46.127.25516,382
18721.46.128.0/1821.46.128.1 to 21.46.191.25421.46.191.25516,382
18821.46.192.0/1821.46.192.1 to 21.46.255.25421.46.255.25516,382
18921.47.0.0/1821.47.0.1 to 21.47.63.25421.47.63.25516,382
19021.47.64.0/1821.47.64.1 to 21.47.127.25421.47.127.25516,382
19121.47.128.0/1821.47.128.1 to 21.47.191.25421.47.191.25516,382
19221.47.192.0/1821.47.192.1 to 21.47.255.25421.47.255.25516,382
19321.48.0.0/1821.48.0.1 to 21.48.63.25421.48.63.25516,382
19421.48.64.0/1821.48.64.1 to 21.48.127.25421.48.127.25516,382
19521.48.128.0/1821.48.128.1 to 21.48.191.25421.48.191.25516,382
19621.48.192.0/1821.48.192.1 to 21.48.255.25421.48.255.25516,382
19721.49.0.0/1821.49.0.1 to 21.49.63.25421.49.63.25516,382
19821.49.64.0/1821.49.64.1 to 21.49.127.25421.49.127.25516,382
19921.49.128.0/1821.49.128.1 to 21.49.191.25421.49.191.25516,382
20021.49.192.0/1821.49.192.1 to 21.49.255.25421.49.255.25516,382
20121.50.0.0/1821.50.0.1 to 21.50.63.25421.50.63.25516,382
20221.50.64.0/1821.50.64.1 to 21.50.127.25421.50.127.25516,382
20321.50.128.0/1821.50.128.1 to 21.50.191.25421.50.191.25516,382
20421.50.192.0/1821.50.192.1 to 21.50.255.25421.50.255.25516,382
20521.51.0.0/1821.51.0.1 to 21.51.63.25421.51.63.25516,382
20621.51.64.0/1821.51.64.1 to 21.51.127.25421.51.127.25516,382
20721.51.128.0/1821.51.128.1 to 21.51.191.25421.51.191.25516,382
20821.51.192.0/1821.51.192.1 to 21.51.255.25421.51.255.25516,382
20921.52.0.0/1821.52.0.1 to 21.52.63.25421.52.63.25516,382
21021.52.64.0/1821.52.64.1 to 21.52.127.25421.52.127.25516,382
21121.52.128.0/1821.52.128.1 to 21.52.191.25421.52.191.25516,382
21221.52.192.0/1821.52.192.1 to 21.52.255.25421.52.255.25516,382
21321.53.0.0/1821.53.0.1 to 21.53.63.25421.53.63.25516,382
21421.53.64.0/1821.53.64.1 to 21.53.127.25421.53.127.25516,382
21521.53.128.0/1821.53.128.1 to 21.53.191.25421.53.191.25516,382
21621.53.192.0/1821.53.192.1 to 21.53.255.25421.53.255.25516,382
21721.54.0.0/1821.54.0.1 to 21.54.63.25421.54.63.25516,382
21821.54.64.0/1821.54.64.1 to 21.54.127.25421.54.127.25516,382
21921.54.128.0/1821.54.128.1 to 21.54.191.25421.54.191.25516,382
22021.54.192.0/1821.54.192.1 to 21.54.255.25421.54.255.25516,382
22121.55.0.0/1821.55.0.1 to 21.55.63.25421.55.63.25516,382
22221.55.64.0/1821.55.64.1 to 21.55.127.25421.55.127.25516,382
22321.55.128.0/1821.55.128.1 to 21.55.191.25421.55.191.25516,382
22421.55.192.0/1821.55.192.1 to 21.55.255.25421.55.255.25516,382
22521.56.0.0/1821.56.0.1 to 21.56.63.25421.56.63.25516,382
22621.56.64.0/1821.56.64.1 to 21.56.127.25421.56.127.25516,382
22721.56.128.0/1821.56.128.1 to 21.56.191.25421.56.191.25516,382
22821.56.192.0/1821.56.192.1 to 21.56.255.25421.56.255.25516,382
22921.57.0.0/1821.57.0.1 to 21.57.63.25421.57.63.25516,382
23021.57.64.0/1821.57.64.1 to 21.57.127.25421.57.127.25516,382
23121.57.128.0/1821.57.128.1 to 21.57.191.25421.57.191.25516,382
23221.57.192.0/1821.57.192.1 to 21.57.255.25421.57.255.25516,382
23321.58.0.0/1821.58.0.1 to 21.58.63.25421.58.63.25516,382
23421.58.64.0/1821.58.64.1 to 21.58.127.25421.58.127.25516,382
23521.58.128.0/1821.58.128.1 to 21.58.191.25421.58.191.25516,382
23621.58.192.0/1821.58.192.1 to 21.58.255.25421.58.255.25516,382
23721.59.0.0/1821.59.0.1 to 21.59.63.25421.59.63.25516,382
23821.59.64.0/1821.59.64.1 to 21.59.127.25421.59.127.25516,382
23921.59.128.0/1821.59.128.1 to 21.59.191.25421.59.191.25516,382
24021.59.192.0/1821.59.192.1 to 21.59.255.25421.59.255.25516,382
24121.60.0.0/1821.60.0.1 to 21.60.63.25421.60.63.25516,382
24221.60.64.0/1821.60.64.1 to 21.60.127.25421.60.127.25516,382
24321.60.128.0/1821.60.128.1 to 21.60.191.25421.60.191.25516,382
24421.60.192.0/1821.60.192.1 to 21.60.255.25421.60.255.25516,382
24521.61.0.0/1821.61.0.1 to 21.61.63.25421.61.63.25516,382
24621.61.64.0/1821.61.64.1 to 21.61.127.25421.61.127.25516,382
24721.61.128.0/1821.61.128.1 to 21.61.191.25421.61.191.25516,382
24821.61.192.0/1821.61.192.1 to 21.61.255.25421.61.255.25516,382
24921.62.0.0/1821.62.0.1 to 21.62.63.25421.62.63.25516,382
25021.62.64.0/1821.62.64.1 to 21.62.127.25421.62.127.25516,382
25121.62.128.0/1821.62.128.1 to 21.62.191.25421.62.191.25516,382
25221.62.192.0/1821.62.192.1 to 21.62.255.25421.62.255.25516,382
25321.63.0.0/1821.63.0.1 to 21.63.63.25421.63.63.25516,382
25421.63.64.0/1821.63.64.1 to 21.63.127.25421.63.127.25516,382
25521.63.128.0/1821.63.128.1 to 21.63.191.25421.63.191.25516,382
25621.63.192.0/1821.63.192.1 to 21.63.255.25421.63.255.25516,382

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.