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.

Prefix/120
Addresses256
/64 networksPart of one
Network
2c19:dd:dd0:ec00:c25:c25:ddfd:dd00/120
First address
2c19:dd:dd0:ec00:c25:c25:ddfd:dd00
Last address
2c19:dd:dd0:ec00:c25:c25:ddfd:ddff
Addresses
256
/64 networks (LANs)
None: smaller than one /64 LAN
Full form
2c19:00dd:0dd0:ec00:0c25:0c25:ddfd:dd00
Type
Global unicast 2000::/3
Public internet addresses
Reverse DNS zone
d.d.d.f.d.d.5.2.c.0.5.2.c.0.0.0.c.e.0.d.d.0.d.d.0.0.9.1.c.2.ip6.arpa
Split into smaller subnets/128
/121 (2)/122 (4)/123 (8)/124 (16)/127 (128)/128 (256)

2c19:dd:dd0:ec00:c25:c25:ddfd:dd00/120 makes 256 subnets of /128.

Subnets
#SubnetLast address
12c19:dd:dd0:ec00:c25:c25:ddfd:dd00/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd00
22c19:dd:dd0:ec00:c25:c25:ddfd:dd01/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd01
32c19:dd:dd0:ec00:c25:c25:ddfd:dd02/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd02
42c19:dd:dd0:ec00:c25:c25:ddfd:dd03/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd03
52c19:dd:dd0:ec00:c25:c25:ddfd:dd04/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd04
62c19:dd:dd0:ec00:c25:c25:ddfd:dd05/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd05
72c19:dd:dd0:ec00:c25:c25:ddfd:dd06/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd06
82c19:dd:dd0:ec00:c25:c25:ddfd:dd07/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd07
92c19:dd:dd0:ec00:c25:c25:ddfd:dd08/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd08
102c19:dd:dd0:ec00:c25:c25:ddfd:dd09/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd09
112c19:dd:dd0:ec00:c25:c25:ddfd:dd0a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd0a
122c19:dd:dd0:ec00:c25:c25:ddfd:dd0b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd0b
132c19:dd:dd0:ec00:c25:c25:ddfd:dd0c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd0c
142c19:dd:dd0:ec00:c25:c25:ddfd:dd0d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd0d
152c19:dd:dd0:ec00:c25:c25:ddfd:dd0e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd0e
162c19:dd:dd0:ec00:c25:c25:ddfd:dd0f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd0f
172c19:dd:dd0:ec00:c25:c25:ddfd:dd10/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd10
182c19:dd:dd0:ec00:c25:c25:ddfd:dd11/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd11
192c19:dd:dd0:ec00:c25:c25:ddfd:dd12/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd12
202c19:dd:dd0:ec00:c25:c25:ddfd:dd13/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd13
212c19:dd:dd0:ec00:c25:c25:ddfd:dd14/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd14
222c19:dd:dd0:ec00:c25:c25:ddfd:dd15/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd15
232c19:dd:dd0:ec00:c25:c25:ddfd:dd16/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd16
242c19:dd:dd0:ec00:c25:c25:ddfd:dd17/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd17
252c19:dd:dd0:ec00:c25:c25:ddfd:dd18/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd18
262c19:dd:dd0:ec00:c25:c25:ddfd:dd19/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd19
272c19:dd:dd0:ec00:c25:c25:ddfd:dd1a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd1a
282c19:dd:dd0:ec00:c25:c25:ddfd:dd1b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd1b
292c19:dd:dd0:ec00:c25:c25:ddfd:dd1c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd1c
302c19:dd:dd0:ec00:c25:c25:ddfd:dd1d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd1d
312c19:dd:dd0:ec00:c25:c25:ddfd:dd1e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd1e
322c19:dd:dd0:ec00:c25:c25:ddfd:dd1f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd1f
332c19:dd:dd0:ec00:c25:c25:ddfd:dd20/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd20
342c19:dd:dd0:ec00:c25:c25:ddfd:dd21/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd21
352c19:dd:dd0:ec00:c25:c25:ddfd:dd22/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd22
362c19:dd:dd0:ec00:c25:c25:ddfd:dd23/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd23
372c19:dd:dd0:ec00:c25:c25:ddfd:dd24/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd24
382c19:dd:dd0:ec00:c25:c25:ddfd:dd25/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd25
392c19:dd:dd0:ec00:c25:c25:ddfd:dd26/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd26
402c19:dd:dd0:ec00:c25:c25:ddfd:dd27/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd27
412c19:dd:dd0:ec00:c25:c25:ddfd:dd28/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd28
422c19:dd:dd0:ec00:c25:c25:ddfd:dd29/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd29
432c19:dd:dd0:ec00:c25:c25:ddfd:dd2a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd2a
442c19:dd:dd0:ec00:c25:c25:ddfd:dd2b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd2b
452c19:dd:dd0:ec00:c25:c25:ddfd:dd2c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd2c
462c19:dd:dd0:ec00:c25:c25:ddfd:dd2d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd2d
472c19:dd:dd0:ec00:c25:c25:ddfd:dd2e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd2e
482c19:dd:dd0:ec00:c25:c25:ddfd:dd2f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd2f
492c19:dd:dd0:ec00:c25:c25:ddfd:dd30/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd30
502c19:dd:dd0:ec00:c25:c25:ddfd:dd31/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd31
512c19:dd:dd0:ec00:c25:c25:ddfd:dd32/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd32
522c19:dd:dd0:ec00:c25:c25:ddfd:dd33/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd33
532c19:dd:dd0:ec00:c25:c25:ddfd:dd34/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd34
542c19:dd:dd0:ec00:c25:c25:ddfd:dd35/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd35
552c19:dd:dd0:ec00:c25:c25:ddfd:dd36/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd36
562c19:dd:dd0:ec00:c25:c25:ddfd:dd37/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd37
572c19:dd:dd0:ec00:c25:c25:ddfd:dd38/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd38
582c19:dd:dd0:ec00:c25:c25:ddfd:dd39/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd39
592c19:dd:dd0:ec00:c25:c25:ddfd:dd3a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd3a
602c19:dd:dd0:ec00:c25:c25:ddfd:dd3b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd3b
612c19:dd:dd0:ec00:c25:c25:ddfd:dd3c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd3c
622c19:dd:dd0:ec00:c25:c25:ddfd:dd3d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd3d
632c19:dd:dd0:ec00:c25:c25:ddfd:dd3e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd3e
642c19:dd:dd0:ec00:c25:c25:ddfd:dd3f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd3f
652c19:dd:dd0:ec00:c25:c25:ddfd:dd40/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd40
662c19:dd:dd0:ec00:c25:c25:ddfd:dd41/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd41
672c19:dd:dd0:ec00:c25:c25:ddfd:dd42/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd42
682c19:dd:dd0:ec00:c25:c25:ddfd:dd43/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd43
692c19:dd:dd0:ec00:c25:c25:ddfd:dd44/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd44
702c19:dd:dd0:ec00:c25:c25:ddfd:dd45/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd45
712c19:dd:dd0:ec00:c25:c25:ddfd:dd46/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd46
722c19:dd:dd0:ec00:c25:c25:ddfd:dd47/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd47
732c19:dd:dd0:ec00:c25:c25:ddfd:dd48/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd48
742c19:dd:dd0:ec00:c25:c25:ddfd:dd49/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd49
752c19:dd:dd0:ec00:c25:c25:ddfd:dd4a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd4a
762c19:dd:dd0:ec00:c25:c25:ddfd:dd4b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd4b
772c19:dd:dd0:ec00:c25:c25:ddfd:dd4c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd4c
782c19:dd:dd0:ec00:c25:c25:ddfd:dd4d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd4d
792c19:dd:dd0:ec00:c25:c25:ddfd:dd4e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd4e
802c19:dd:dd0:ec00:c25:c25:ddfd:dd4f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd4f
812c19:dd:dd0:ec00:c25:c25:ddfd:dd50/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd50
822c19:dd:dd0:ec00:c25:c25:ddfd:dd51/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd51
832c19:dd:dd0:ec00:c25:c25:ddfd:dd52/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd52
842c19:dd:dd0:ec00:c25:c25:ddfd:dd53/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd53
852c19:dd:dd0:ec00:c25:c25:ddfd:dd54/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd54
862c19:dd:dd0:ec00:c25:c25:ddfd:dd55/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd55
872c19:dd:dd0:ec00:c25:c25:ddfd:dd56/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd56
882c19:dd:dd0:ec00:c25:c25:ddfd:dd57/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd57
892c19:dd:dd0:ec00:c25:c25:ddfd:dd58/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd58
902c19:dd:dd0:ec00:c25:c25:ddfd:dd59/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd59
912c19:dd:dd0:ec00:c25:c25:ddfd:dd5a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd5a
922c19:dd:dd0:ec00:c25:c25:ddfd:dd5b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd5b
932c19:dd:dd0:ec00:c25:c25:ddfd:dd5c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd5c
942c19:dd:dd0:ec00:c25:c25:ddfd:dd5d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd5d
952c19:dd:dd0:ec00:c25:c25:ddfd:dd5e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd5e
962c19:dd:dd0:ec00:c25:c25:ddfd:dd5f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd5f
972c19:dd:dd0:ec00:c25:c25:ddfd:dd60/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd60
982c19:dd:dd0:ec00:c25:c25:ddfd:dd61/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd61
992c19:dd:dd0:ec00:c25:c25:ddfd:dd62/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd62
1002c19:dd:dd0:ec00:c25:c25:ddfd:dd63/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd63
1012c19:dd:dd0:ec00:c25:c25:ddfd:dd64/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd64
1022c19:dd:dd0:ec00:c25:c25:ddfd:dd65/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd65
1032c19:dd:dd0:ec00:c25:c25:ddfd:dd66/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd66
1042c19:dd:dd0:ec00:c25:c25:ddfd:dd67/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd67
1052c19:dd:dd0:ec00:c25:c25:ddfd:dd68/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd68
1062c19:dd:dd0:ec00:c25:c25:ddfd:dd69/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd69
1072c19:dd:dd0:ec00:c25:c25:ddfd:dd6a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd6a
1082c19:dd:dd0:ec00:c25:c25:ddfd:dd6b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd6b
1092c19:dd:dd0:ec00:c25:c25:ddfd:dd6c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd6c
1102c19:dd:dd0:ec00:c25:c25:ddfd:dd6d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd6d
1112c19:dd:dd0:ec00:c25:c25:ddfd:dd6e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd6e
1122c19:dd:dd0:ec00:c25:c25:ddfd:dd6f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd6f
1132c19:dd:dd0:ec00:c25:c25:ddfd:dd70/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd70
1142c19:dd:dd0:ec00:c25:c25:ddfd:dd71/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd71
1152c19:dd:dd0:ec00:c25:c25:ddfd:dd72/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd72
1162c19:dd:dd0:ec00:c25:c25:ddfd:dd73/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd73
1172c19:dd:dd0:ec00:c25:c25:ddfd:dd74/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd74
1182c19:dd:dd0:ec00:c25:c25:ddfd:dd75/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd75
1192c19:dd:dd0:ec00:c25:c25:ddfd:dd76/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd76
1202c19:dd:dd0:ec00:c25:c25:ddfd:dd77/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd77
1212c19:dd:dd0:ec00:c25:c25:ddfd:dd78/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd78
1222c19:dd:dd0:ec00:c25:c25:ddfd:dd79/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd79
1232c19:dd:dd0:ec00:c25:c25:ddfd:dd7a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd7a
1242c19:dd:dd0:ec00:c25:c25:ddfd:dd7b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd7b
1252c19:dd:dd0:ec00:c25:c25:ddfd:dd7c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd7c
1262c19:dd:dd0:ec00:c25:c25:ddfd:dd7d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd7d
1272c19:dd:dd0:ec00:c25:c25:ddfd:dd7e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd7e
1282c19:dd:dd0:ec00:c25:c25:ddfd:dd7f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd7f
1292c19:dd:dd0:ec00:c25:c25:ddfd:dd80/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd80
1302c19:dd:dd0:ec00:c25:c25:ddfd:dd81/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd81
1312c19:dd:dd0:ec00:c25:c25:ddfd:dd82/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd82
1322c19:dd:dd0:ec00:c25:c25:ddfd:dd83/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd83
1332c19:dd:dd0:ec00:c25:c25:ddfd:dd84/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd84
1342c19:dd:dd0:ec00:c25:c25:ddfd:dd85/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd85
1352c19:dd:dd0:ec00:c25:c25:ddfd:dd86/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd86
1362c19:dd:dd0:ec00:c25:c25:ddfd:dd87/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd87
1372c19:dd:dd0:ec00:c25:c25:ddfd:dd88/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd88
1382c19:dd:dd0:ec00:c25:c25:ddfd:dd89/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd89
1392c19:dd:dd0:ec00:c25:c25:ddfd:dd8a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd8a
1402c19:dd:dd0:ec00:c25:c25:ddfd:dd8b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd8b
1412c19:dd:dd0:ec00:c25:c25:ddfd:dd8c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd8c
1422c19:dd:dd0:ec00:c25:c25:ddfd:dd8d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd8d
1432c19:dd:dd0:ec00:c25:c25:ddfd:dd8e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd8e
1442c19:dd:dd0:ec00:c25:c25:ddfd:dd8f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd8f
1452c19:dd:dd0:ec00:c25:c25:ddfd:dd90/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd90
1462c19:dd:dd0:ec00:c25:c25:ddfd:dd91/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd91
1472c19:dd:dd0:ec00:c25:c25:ddfd:dd92/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd92
1482c19:dd:dd0:ec00:c25:c25:ddfd:dd93/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd93
1492c19:dd:dd0:ec00:c25:c25:ddfd:dd94/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd94
1502c19:dd:dd0:ec00:c25:c25:ddfd:dd95/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd95
1512c19:dd:dd0:ec00:c25:c25:ddfd:dd96/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd96
1522c19:dd:dd0:ec00:c25:c25:ddfd:dd97/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd97
1532c19:dd:dd0:ec00:c25:c25:ddfd:dd98/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd98
1542c19:dd:dd0:ec00:c25:c25:ddfd:dd99/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd99
1552c19:dd:dd0:ec00:c25:c25:ddfd:dd9a/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd9a
1562c19:dd:dd0:ec00:c25:c25:ddfd:dd9b/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd9b
1572c19:dd:dd0:ec00:c25:c25:ddfd:dd9c/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd9c
1582c19:dd:dd0:ec00:c25:c25:ddfd:dd9d/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd9d
1592c19:dd:dd0:ec00:c25:c25:ddfd:dd9e/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd9e
1602c19:dd:dd0:ec00:c25:c25:ddfd:dd9f/1282c19:dd:dd0:ec00:c25:c25:ddfd:dd9f
1612c19:dd:dd0:ec00:c25:c25:ddfd:dda0/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda0
1622c19:dd:dd0:ec00:c25:c25:ddfd:dda1/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda1
1632c19:dd:dd0:ec00:c25:c25:ddfd:dda2/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda2
1642c19:dd:dd0:ec00:c25:c25:ddfd:dda3/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda3
1652c19:dd:dd0:ec00:c25:c25:ddfd:dda4/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda4
1662c19:dd:dd0:ec00:c25:c25:ddfd:dda5/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda5
1672c19:dd:dd0:ec00:c25:c25:ddfd:dda6/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda6
1682c19:dd:dd0:ec00:c25:c25:ddfd:dda7/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda7
1692c19:dd:dd0:ec00:c25:c25:ddfd:dda8/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda8
1702c19:dd:dd0:ec00:c25:c25:ddfd:dda9/1282c19:dd:dd0:ec00:c25:c25:ddfd:dda9
1712c19:dd:dd0:ec00:c25:c25:ddfd:ddaa/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddaa
1722c19:dd:dd0:ec00:c25:c25:ddfd:ddab/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddab
1732c19:dd:dd0:ec00:c25:c25:ddfd:ddac/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddac
1742c19:dd:dd0:ec00:c25:c25:ddfd:ddad/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddad
1752c19:dd:dd0:ec00:c25:c25:ddfd:ddae/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddae
1762c19:dd:dd0:ec00:c25:c25:ddfd:ddaf/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddaf
1772c19:dd:dd0:ec00:c25:c25:ddfd:ddb0/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb0
1782c19:dd:dd0:ec00:c25:c25:ddfd:ddb1/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb1
1792c19:dd:dd0:ec00:c25:c25:ddfd:ddb2/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb2
1802c19:dd:dd0:ec00:c25:c25:ddfd:ddb3/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb3
1812c19:dd:dd0:ec00:c25:c25:ddfd:ddb4/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb4
1822c19:dd:dd0:ec00:c25:c25:ddfd:ddb5/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb5
1832c19:dd:dd0:ec00:c25:c25:ddfd:ddb6/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb6
1842c19:dd:dd0:ec00:c25:c25:ddfd:ddb7/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb7
1852c19:dd:dd0:ec00:c25:c25:ddfd:ddb8/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb8
1862c19:dd:dd0:ec00:c25:c25:ddfd:ddb9/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddb9
1872c19:dd:dd0:ec00:c25:c25:ddfd:ddba/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddba
1882c19:dd:dd0:ec00:c25:c25:ddfd:ddbb/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddbb
1892c19:dd:dd0:ec00:c25:c25:ddfd:ddbc/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddbc
1902c19:dd:dd0:ec00:c25:c25:ddfd:ddbd/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddbd
1912c19:dd:dd0:ec00:c25:c25:ddfd:ddbe/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddbe
1922c19:dd:dd0:ec00:c25:c25:ddfd:ddbf/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddbf
1932c19:dd:dd0:ec00:c25:c25:ddfd:ddc0/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc0
1942c19:dd:dd0:ec00:c25:c25:ddfd:ddc1/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc1
1952c19:dd:dd0:ec00:c25:c25:ddfd:ddc2/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc2
1962c19:dd:dd0:ec00:c25:c25:ddfd:ddc3/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc3
1972c19:dd:dd0:ec00:c25:c25:ddfd:ddc4/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc4
1982c19:dd:dd0:ec00:c25:c25:ddfd:ddc5/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc5
1992c19:dd:dd0:ec00:c25:c25:ddfd:ddc6/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc6
2002c19:dd:dd0:ec00:c25:c25:ddfd:ddc7/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc7
2012c19:dd:dd0:ec00:c25:c25:ddfd:ddc8/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc8
2022c19:dd:dd0:ec00:c25:c25:ddfd:ddc9/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddc9
2032c19:dd:dd0:ec00:c25:c25:ddfd:ddca/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddca
2042c19:dd:dd0:ec00:c25:c25:ddfd:ddcb/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddcb
2052c19:dd:dd0:ec00:c25:c25:ddfd:ddcc/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddcc
2062c19:dd:dd0:ec00:c25:c25:ddfd:ddcd/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddcd
2072c19:dd:dd0:ec00:c25:c25:ddfd:ddce/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddce
2082c19:dd:dd0:ec00:c25:c25:ddfd:ddcf/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddcf
2092c19:dd:dd0:ec00:c25:c25:ddfd:ddd0/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd0
2102c19:dd:dd0:ec00:c25:c25:ddfd:ddd1/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd1
2112c19:dd:dd0:ec00:c25:c25:ddfd:ddd2/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd2
2122c19:dd:dd0:ec00:c25:c25:ddfd:ddd3/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd3
2132c19:dd:dd0:ec00:c25:c25:ddfd:ddd4/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd4
2142c19:dd:dd0:ec00:c25:c25:ddfd:ddd5/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd5
2152c19:dd:dd0:ec00:c25:c25:ddfd:ddd6/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd6
2162c19:dd:dd0:ec00:c25:c25:ddfd:ddd7/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd7
2172c19:dd:dd0:ec00:c25:c25:ddfd:ddd8/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd8
2182c19:dd:dd0:ec00:c25:c25:ddfd:ddd9/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddd9
2192c19:dd:dd0:ec00:c25:c25:ddfd:ddda/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddda
2202c19:dd:dd0:ec00:c25:c25:ddfd:dddb/1282c19:dd:dd0:ec00:c25:c25:ddfd:dddb
2212c19:dd:dd0:ec00:c25:c25:ddfd:dddc/1282c19:dd:dd0:ec00:c25:c25:ddfd:dddc
2222c19:dd:dd0:ec00:c25:c25:ddfd:dddd/1282c19:dd:dd0:ec00:c25:c25:ddfd:dddd
2232c19:dd:dd0:ec00:c25:c25:ddfd:ddde/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddde
2242c19:dd:dd0:ec00:c25:c25:ddfd:dddf/1282c19:dd:dd0:ec00:c25:c25:ddfd:dddf
2252c19:dd:dd0:ec00:c25:c25:ddfd:dde0/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde0
2262c19:dd:dd0:ec00:c25:c25:ddfd:dde1/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde1
2272c19:dd:dd0:ec00:c25:c25:ddfd:dde2/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde2
2282c19:dd:dd0:ec00:c25:c25:ddfd:dde3/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde3
2292c19:dd:dd0:ec00:c25:c25:ddfd:dde4/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde4
2302c19:dd:dd0:ec00:c25:c25:ddfd:dde5/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde5
2312c19:dd:dd0:ec00:c25:c25:ddfd:dde6/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde6
2322c19:dd:dd0:ec00:c25:c25:ddfd:dde7/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde7
2332c19:dd:dd0:ec00:c25:c25:ddfd:dde8/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde8
2342c19:dd:dd0:ec00:c25:c25:ddfd:dde9/1282c19:dd:dd0:ec00:c25:c25:ddfd:dde9
2352c19:dd:dd0:ec00:c25:c25:ddfd:ddea/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddea
2362c19:dd:dd0:ec00:c25:c25:ddfd:ddeb/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddeb
2372c19:dd:dd0:ec00:c25:c25:ddfd:ddec/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddec
2382c19:dd:dd0:ec00:c25:c25:ddfd:dded/1282c19:dd:dd0:ec00:c25:c25:ddfd:dded
2392c19:dd:dd0:ec00:c25:c25:ddfd:ddee/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddee
2402c19:dd:dd0:ec00:c25:c25:ddfd:ddef/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddef
2412c19:dd:dd0:ec00:c25:c25:ddfd:ddf0/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf0
2422c19:dd:dd0:ec00:c25:c25:ddfd:ddf1/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf1
2432c19:dd:dd0:ec00:c25:c25:ddfd:ddf2/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf2
2442c19:dd:dd0:ec00:c25:c25:ddfd:ddf3/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf3
2452c19:dd:dd0:ec00:c25:c25:ddfd:ddf4/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf4
2462c19:dd:dd0:ec00:c25:c25:ddfd:ddf5/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf5
2472c19:dd:dd0:ec00:c25:c25:ddfd:ddf6/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf6
2482c19:dd:dd0:ec00:c25:c25:ddfd:ddf7/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf7
2492c19:dd:dd0:ec00:c25:c25:ddfd:ddf8/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf8
2502c19:dd:dd0:ec00:c25:c25:ddfd:ddf9/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddf9
2512c19:dd:dd0:ec00:c25:c25:ddfd:ddfa/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddfa
2522c19:dd:dd0:ec00:c25:c25:ddfd:ddfb/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddfb
2532c19:dd:dd0:ec00:c25:c25:ddfd:ddfc/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddfc
2542c19:dd:dd0:ec00:c25:c25:ddfd:ddfd/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddfd
2552c19:dd:dd0:ec00:c25:c25:ddfd:ddfe/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddfe
2562c19:dd:dd0:ec00:c25:c25:ddfd:ddff/1282c19:dd:dd0:ec00:c25:c25:ddfd:ddff

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.