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:ec00:c25:c25e:dd:ddbb:cb00/120
First address
2c19:dd:ec00:c25:c25e:dd:ddbb:cb00
Last address
2c19:dd:ec00:c25:c25e:dd:ddbb:cbff
Addresses
256
/64 networks (LANs)
None: smaller than one /64 LAN
Full form
2c19:00dd:ec00:0c25:c25e:00dd:ddbb:cb00
Type
Global unicast 2000::/3
Public internet addresses
Reverse DNS zone
b.c.b.b.d.d.d.d.0.0.e.5.2.c.5.2.c.0.0.0.c.e.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:ec00:c25:c25e:dd:ddbb:cb00/120 makes 256 subnets of /128.

Subnets
#SubnetLast address
12c19:dd:ec00:c25:c25e:dd:ddbb:cb00/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb00
22c19:dd:ec00:c25:c25e:dd:ddbb:cb01/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb01
32c19:dd:ec00:c25:c25e:dd:ddbb:cb02/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb02
42c19:dd:ec00:c25:c25e:dd:ddbb:cb03/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb03
52c19:dd:ec00:c25:c25e:dd:ddbb:cb04/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb04
62c19:dd:ec00:c25:c25e:dd:ddbb:cb05/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb05
72c19:dd:ec00:c25:c25e:dd:ddbb:cb06/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb06
82c19:dd:ec00:c25:c25e:dd:ddbb:cb07/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb07
92c19:dd:ec00:c25:c25e:dd:ddbb:cb08/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb08
102c19:dd:ec00:c25:c25e:dd:ddbb:cb09/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb09
112c19:dd:ec00:c25:c25e:dd:ddbb:cb0a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb0a
122c19:dd:ec00:c25:c25e:dd:ddbb:cb0b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb0b
132c19:dd:ec00:c25:c25e:dd:ddbb:cb0c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb0c
142c19:dd:ec00:c25:c25e:dd:ddbb:cb0d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb0d
152c19:dd:ec00:c25:c25e:dd:ddbb:cb0e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb0e
162c19:dd:ec00:c25:c25e:dd:ddbb:cb0f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb0f
172c19:dd:ec00:c25:c25e:dd:ddbb:cb10/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb10
182c19:dd:ec00:c25:c25e:dd:ddbb:cb11/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb11
192c19:dd:ec00:c25:c25e:dd:ddbb:cb12/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb12
202c19:dd:ec00:c25:c25e:dd:ddbb:cb13/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb13
212c19:dd:ec00:c25:c25e:dd:ddbb:cb14/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb14
222c19:dd:ec00:c25:c25e:dd:ddbb:cb15/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb15
232c19:dd:ec00:c25:c25e:dd:ddbb:cb16/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb16
242c19:dd:ec00:c25:c25e:dd:ddbb:cb17/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb17
252c19:dd:ec00:c25:c25e:dd:ddbb:cb18/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb18
262c19:dd:ec00:c25:c25e:dd:ddbb:cb19/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb19
272c19:dd:ec00:c25:c25e:dd:ddbb:cb1a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb1a
282c19:dd:ec00:c25:c25e:dd:ddbb:cb1b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb1b
292c19:dd:ec00:c25:c25e:dd:ddbb:cb1c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb1c
302c19:dd:ec00:c25:c25e:dd:ddbb:cb1d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb1d
312c19:dd:ec00:c25:c25e:dd:ddbb:cb1e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb1e
322c19:dd:ec00:c25:c25e:dd:ddbb:cb1f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb1f
332c19:dd:ec00:c25:c25e:dd:ddbb:cb20/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb20
342c19:dd:ec00:c25:c25e:dd:ddbb:cb21/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb21
352c19:dd:ec00:c25:c25e:dd:ddbb:cb22/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb22
362c19:dd:ec00:c25:c25e:dd:ddbb:cb23/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb23
372c19:dd:ec00:c25:c25e:dd:ddbb:cb24/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb24
382c19:dd:ec00:c25:c25e:dd:ddbb:cb25/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb25
392c19:dd:ec00:c25:c25e:dd:ddbb:cb26/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb26
402c19:dd:ec00:c25:c25e:dd:ddbb:cb27/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb27
412c19:dd:ec00:c25:c25e:dd:ddbb:cb28/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb28
422c19:dd:ec00:c25:c25e:dd:ddbb:cb29/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb29
432c19:dd:ec00:c25:c25e:dd:ddbb:cb2a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb2a
442c19:dd:ec00:c25:c25e:dd:ddbb:cb2b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb2b
452c19:dd:ec00:c25:c25e:dd:ddbb:cb2c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb2c
462c19:dd:ec00:c25:c25e:dd:ddbb:cb2d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb2d
472c19:dd:ec00:c25:c25e:dd:ddbb:cb2e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb2e
482c19:dd:ec00:c25:c25e:dd:ddbb:cb2f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb2f
492c19:dd:ec00:c25:c25e:dd:ddbb:cb30/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb30
502c19:dd:ec00:c25:c25e:dd:ddbb:cb31/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb31
512c19:dd:ec00:c25:c25e:dd:ddbb:cb32/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb32
522c19:dd:ec00:c25:c25e:dd:ddbb:cb33/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb33
532c19:dd:ec00:c25:c25e:dd:ddbb:cb34/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb34
542c19:dd:ec00:c25:c25e:dd:ddbb:cb35/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb35
552c19:dd:ec00:c25:c25e:dd:ddbb:cb36/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb36
562c19:dd:ec00:c25:c25e:dd:ddbb:cb37/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb37
572c19:dd:ec00:c25:c25e:dd:ddbb:cb38/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb38
582c19:dd:ec00:c25:c25e:dd:ddbb:cb39/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb39
592c19:dd:ec00:c25:c25e:dd:ddbb:cb3a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb3a
602c19:dd:ec00:c25:c25e:dd:ddbb:cb3b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb3b
612c19:dd:ec00:c25:c25e:dd:ddbb:cb3c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb3c
622c19:dd:ec00:c25:c25e:dd:ddbb:cb3d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb3d
632c19:dd:ec00:c25:c25e:dd:ddbb:cb3e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb3e
642c19:dd:ec00:c25:c25e:dd:ddbb:cb3f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb3f
652c19:dd:ec00:c25:c25e:dd:ddbb:cb40/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb40
662c19:dd:ec00:c25:c25e:dd:ddbb:cb41/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb41
672c19:dd:ec00:c25:c25e:dd:ddbb:cb42/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb42
682c19:dd:ec00:c25:c25e:dd:ddbb:cb43/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb43
692c19:dd:ec00:c25:c25e:dd:ddbb:cb44/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb44
702c19:dd:ec00:c25:c25e:dd:ddbb:cb45/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb45
712c19:dd:ec00:c25:c25e:dd:ddbb:cb46/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb46
722c19:dd:ec00:c25:c25e:dd:ddbb:cb47/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb47
732c19:dd:ec00:c25:c25e:dd:ddbb:cb48/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb48
742c19:dd:ec00:c25:c25e:dd:ddbb:cb49/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb49
752c19:dd:ec00:c25:c25e:dd:ddbb:cb4a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb4a
762c19:dd:ec00:c25:c25e:dd:ddbb:cb4b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb4b
772c19:dd:ec00:c25:c25e:dd:ddbb:cb4c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb4c
782c19:dd:ec00:c25:c25e:dd:ddbb:cb4d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb4d
792c19:dd:ec00:c25:c25e:dd:ddbb:cb4e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb4e
802c19:dd:ec00:c25:c25e:dd:ddbb:cb4f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb4f
812c19:dd:ec00:c25:c25e:dd:ddbb:cb50/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb50
822c19:dd:ec00:c25:c25e:dd:ddbb:cb51/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb51
832c19:dd:ec00:c25:c25e:dd:ddbb:cb52/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb52
842c19:dd:ec00:c25:c25e:dd:ddbb:cb53/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb53
852c19:dd:ec00:c25:c25e:dd:ddbb:cb54/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb54
862c19:dd:ec00:c25:c25e:dd:ddbb:cb55/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb55
872c19:dd:ec00:c25:c25e:dd:ddbb:cb56/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb56
882c19:dd:ec00:c25:c25e:dd:ddbb:cb57/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb57
892c19:dd:ec00:c25:c25e:dd:ddbb:cb58/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb58
902c19:dd:ec00:c25:c25e:dd:ddbb:cb59/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb59
912c19:dd:ec00:c25:c25e:dd:ddbb:cb5a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb5a
922c19:dd:ec00:c25:c25e:dd:ddbb:cb5b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb5b
932c19:dd:ec00:c25:c25e:dd:ddbb:cb5c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb5c
942c19:dd:ec00:c25:c25e:dd:ddbb:cb5d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb5d
952c19:dd:ec00:c25:c25e:dd:ddbb:cb5e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb5e
962c19:dd:ec00:c25:c25e:dd:ddbb:cb5f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb5f
972c19:dd:ec00:c25:c25e:dd:ddbb:cb60/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb60
982c19:dd:ec00:c25:c25e:dd:ddbb:cb61/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb61
992c19:dd:ec00:c25:c25e:dd:ddbb:cb62/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb62
1002c19:dd:ec00:c25:c25e:dd:ddbb:cb63/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb63
1012c19:dd:ec00:c25:c25e:dd:ddbb:cb64/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb64
1022c19:dd:ec00:c25:c25e:dd:ddbb:cb65/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb65
1032c19:dd:ec00:c25:c25e:dd:ddbb:cb66/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb66
1042c19:dd:ec00:c25:c25e:dd:ddbb:cb67/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb67
1052c19:dd:ec00:c25:c25e:dd:ddbb:cb68/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb68
1062c19:dd:ec00:c25:c25e:dd:ddbb:cb69/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb69
1072c19:dd:ec00:c25:c25e:dd:ddbb:cb6a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb6a
1082c19:dd:ec00:c25:c25e:dd:ddbb:cb6b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb6b
1092c19:dd:ec00:c25:c25e:dd:ddbb:cb6c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb6c
1102c19:dd:ec00:c25:c25e:dd:ddbb:cb6d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb6d
1112c19:dd:ec00:c25:c25e:dd:ddbb:cb6e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb6e
1122c19:dd:ec00:c25:c25e:dd:ddbb:cb6f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb6f
1132c19:dd:ec00:c25:c25e:dd:ddbb:cb70/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb70
1142c19:dd:ec00:c25:c25e:dd:ddbb:cb71/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb71
1152c19:dd:ec00:c25:c25e:dd:ddbb:cb72/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb72
1162c19:dd:ec00:c25:c25e:dd:ddbb:cb73/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb73
1172c19:dd:ec00:c25:c25e:dd:ddbb:cb74/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb74
1182c19:dd:ec00:c25:c25e:dd:ddbb:cb75/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb75
1192c19:dd:ec00:c25:c25e:dd:ddbb:cb76/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb76
1202c19:dd:ec00:c25:c25e:dd:ddbb:cb77/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb77
1212c19:dd:ec00:c25:c25e:dd:ddbb:cb78/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb78
1222c19:dd:ec00:c25:c25e:dd:ddbb:cb79/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb79
1232c19:dd:ec00:c25:c25e:dd:ddbb:cb7a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb7a
1242c19:dd:ec00:c25:c25e:dd:ddbb:cb7b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb7b
1252c19:dd:ec00:c25:c25e:dd:ddbb:cb7c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb7c
1262c19:dd:ec00:c25:c25e:dd:ddbb:cb7d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb7d
1272c19:dd:ec00:c25:c25e:dd:ddbb:cb7e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb7e
1282c19:dd:ec00:c25:c25e:dd:ddbb:cb7f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb7f
1292c19:dd:ec00:c25:c25e:dd:ddbb:cb80/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb80
1302c19:dd:ec00:c25:c25e:dd:ddbb:cb81/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb81
1312c19:dd:ec00:c25:c25e:dd:ddbb:cb82/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb82
1322c19:dd:ec00:c25:c25e:dd:ddbb:cb83/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb83
1332c19:dd:ec00:c25:c25e:dd:ddbb:cb84/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb84
1342c19:dd:ec00:c25:c25e:dd:ddbb:cb85/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb85
1352c19:dd:ec00:c25:c25e:dd:ddbb:cb86/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb86
1362c19:dd:ec00:c25:c25e:dd:ddbb:cb87/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb87
1372c19:dd:ec00:c25:c25e:dd:ddbb:cb88/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb88
1382c19:dd:ec00:c25:c25e:dd:ddbb:cb89/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb89
1392c19:dd:ec00:c25:c25e:dd:ddbb:cb8a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb8a
1402c19:dd:ec00:c25:c25e:dd:ddbb:cb8b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb8b
1412c19:dd:ec00:c25:c25e:dd:ddbb:cb8c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb8c
1422c19:dd:ec00:c25:c25e:dd:ddbb:cb8d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb8d
1432c19:dd:ec00:c25:c25e:dd:ddbb:cb8e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb8e
1442c19:dd:ec00:c25:c25e:dd:ddbb:cb8f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb8f
1452c19:dd:ec00:c25:c25e:dd:ddbb:cb90/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb90
1462c19:dd:ec00:c25:c25e:dd:ddbb:cb91/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb91
1472c19:dd:ec00:c25:c25e:dd:ddbb:cb92/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb92
1482c19:dd:ec00:c25:c25e:dd:ddbb:cb93/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb93
1492c19:dd:ec00:c25:c25e:dd:ddbb:cb94/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb94
1502c19:dd:ec00:c25:c25e:dd:ddbb:cb95/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb95
1512c19:dd:ec00:c25:c25e:dd:ddbb:cb96/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb96
1522c19:dd:ec00:c25:c25e:dd:ddbb:cb97/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb97
1532c19:dd:ec00:c25:c25e:dd:ddbb:cb98/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb98
1542c19:dd:ec00:c25:c25e:dd:ddbb:cb99/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb99
1552c19:dd:ec00:c25:c25e:dd:ddbb:cb9a/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb9a
1562c19:dd:ec00:c25:c25e:dd:ddbb:cb9b/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb9b
1572c19:dd:ec00:c25:c25e:dd:ddbb:cb9c/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb9c
1582c19:dd:ec00:c25:c25e:dd:ddbb:cb9d/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb9d
1592c19:dd:ec00:c25:c25e:dd:ddbb:cb9e/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb9e
1602c19:dd:ec00:c25:c25e:dd:ddbb:cb9f/1282c19:dd:ec00:c25:c25e:dd:ddbb:cb9f
1612c19:dd:ec00:c25:c25e:dd:ddbb:cba0/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba0
1622c19:dd:ec00:c25:c25e:dd:ddbb:cba1/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba1
1632c19:dd:ec00:c25:c25e:dd:ddbb:cba2/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba2
1642c19:dd:ec00:c25:c25e:dd:ddbb:cba3/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba3
1652c19:dd:ec00:c25:c25e:dd:ddbb:cba4/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba4
1662c19:dd:ec00:c25:c25e:dd:ddbb:cba5/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba5
1672c19:dd:ec00:c25:c25e:dd:ddbb:cba6/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba6
1682c19:dd:ec00:c25:c25e:dd:ddbb:cba7/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba7
1692c19:dd:ec00:c25:c25e:dd:ddbb:cba8/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba8
1702c19:dd:ec00:c25:c25e:dd:ddbb:cba9/1282c19:dd:ec00:c25:c25e:dd:ddbb:cba9
1712c19:dd:ec00:c25:c25e:dd:ddbb:cbaa/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbaa
1722c19:dd:ec00:c25:c25e:dd:ddbb:cbab/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbab
1732c19:dd:ec00:c25:c25e:dd:ddbb:cbac/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbac
1742c19:dd:ec00:c25:c25e:dd:ddbb:cbad/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbad
1752c19:dd:ec00:c25:c25e:dd:ddbb:cbae/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbae
1762c19:dd:ec00:c25:c25e:dd:ddbb:cbaf/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbaf
1772c19:dd:ec00:c25:c25e:dd:ddbb:cbb0/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb0
1782c19:dd:ec00:c25:c25e:dd:ddbb:cbb1/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb1
1792c19:dd:ec00:c25:c25e:dd:ddbb:cbb2/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb2
1802c19:dd:ec00:c25:c25e:dd:ddbb:cbb3/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb3
1812c19:dd:ec00:c25:c25e:dd:ddbb:cbb4/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb4
1822c19:dd:ec00:c25:c25e:dd:ddbb:cbb5/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb5
1832c19:dd:ec00:c25:c25e:dd:ddbb:cbb6/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb6
1842c19:dd:ec00:c25:c25e:dd:ddbb:cbb7/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb7
1852c19:dd:ec00:c25:c25e:dd:ddbb:cbb8/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb8
1862c19:dd:ec00:c25:c25e:dd:ddbb:cbb9/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbb9
1872c19:dd:ec00:c25:c25e:dd:ddbb:cbba/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbba
1882c19:dd:ec00:c25:c25e:dd:ddbb:cbbb/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbbb
1892c19:dd:ec00:c25:c25e:dd:ddbb:cbbc/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbbc
1902c19:dd:ec00:c25:c25e:dd:ddbb:cbbd/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbbd
1912c19:dd:ec00:c25:c25e:dd:ddbb:cbbe/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbbe
1922c19:dd:ec00:c25:c25e:dd:ddbb:cbbf/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbbf
1932c19:dd:ec00:c25:c25e:dd:ddbb:cbc0/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc0
1942c19:dd:ec00:c25:c25e:dd:ddbb:cbc1/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc1
1952c19:dd:ec00:c25:c25e:dd:ddbb:cbc2/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc2
1962c19:dd:ec00:c25:c25e:dd:ddbb:cbc3/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc3
1972c19:dd:ec00:c25:c25e:dd:ddbb:cbc4/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc4
1982c19:dd:ec00:c25:c25e:dd:ddbb:cbc5/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc5
1992c19:dd:ec00:c25:c25e:dd:ddbb:cbc6/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc6
2002c19:dd:ec00:c25:c25e:dd:ddbb:cbc7/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc7
2012c19:dd:ec00:c25:c25e:dd:ddbb:cbc8/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc8
2022c19:dd:ec00:c25:c25e:dd:ddbb:cbc9/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbc9
2032c19:dd:ec00:c25:c25e:dd:ddbb:cbca/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbca
2042c19:dd:ec00:c25:c25e:dd:ddbb:cbcb/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbcb
2052c19:dd:ec00:c25:c25e:dd:ddbb:cbcc/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbcc
2062c19:dd:ec00:c25:c25e:dd:ddbb:cbcd/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbcd
2072c19:dd:ec00:c25:c25e:dd:ddbb:cbce/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbce
2082c19:dd:ec00:c25:c25e:dd:ddbb:cbcf/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbcf
2092c19:dd:ec00:c25:c25e:dd:ddbb:cbd0/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd0
2102c19:dd:ec00:c25:c25e:dd:ddbb:cbd1/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd1
2112c19:dd:ec00:c25:c25e:dd:ddbb:cbd2/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd2
2122c19:dd:ec00:c25:c25e:dd:ddbb:cbd3/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd3
2132c19:dd:ec00:c25:c25e:dd:ddbb:cbd4/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd4
2142c19:dd:ec00:c25:c25e:dd:ddbb:cbd5/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd5
2152c19:dd:ec00:c25:c25e:dd:ddbb:cbd6/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd6
2162c19:dd:ec00:c25:c25e:dd:ddbb:cbd7/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd7
2172c19:dd:ec00:c25:c25e:dd:ddbb:cbd8/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd8
2182c19:dd:ec00:c25:c25e:dd:ddbb:cbd9/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbd9
2192c19:dd:ec00:c25:c25e:dd:ddbb:cbda/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbda
2202c19:dd:ec00:c25:c25e:dd:ddbb:cbdb/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbdb
2212c19:dd:ec00:c25:c25e:dd:ddbb:cbdc/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbdc
2222c19:dd:ec00:c25:c25e:dd:ddbb:cbdd/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbdd
2232c19:dd:ec00:c25:c25e:dd:ddbb:cbde/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbde
2242c19:dd:ec00:c25:c25e:dd:ddbb:cbdf/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbdf
2252c19:dd:ec00:c25:c25e:dd:ddbb:cbe0/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe0
2262c19:dd:ec00:c25:c25e:dd:ddbb:cbe1/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe1
2272c19:dd:ec00:c25:c25e:dd:ddbb:cbe2/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe2
2282c19:dd:ec00:c25:c25e:dd:ddbb:cbe3/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe3
2292c19:dd:ec00:c25:c25e:dd:ddbb:cbe4/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe4
2302c19:dd:ec00:c25:c25e:dd:ddbb:cbe5/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe5
2312c19:dd:ec00:c25:c25e:dd:ddbb:cbe6/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe6
2322c19:dd:ec00:c25:c25e:dd:ddbb:cbe7/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe7
2332c19:dd:ec00:c25:c25e:dd:ddbb:cbe8/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe8
2342c19:dd:ec00:c25:c25e:dd:ddbb:cbe9/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbe9
2352c19:dd:ec00:c25:c25e:dd:ddbb:cbea/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbea
2362c19:dd:ec00:c25:c25e:dd:ddbb:cbeb/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbeb
2372c19:dd:ec00:c25:c25e:dd:ddbb:cbec/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbec
2382c19:dd:ec00:c25:c25e:dd:ddbb:cbed/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbed
2392c19:dd:ec00:c25:c25e:dd:ddbb:cbee/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbee
2402c19:dd:ec00:c25:c25e:dd:ddbb:cbef/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbef
2412c19:dd:ec00:c25:c25e:dd:ddbb:cbf0/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf0
2422c19:dd:ec00:c25:c25e:dd:ddbb:cbf1/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf1
2432c19:dd:ec00:c25:c25e:dd:ddbb:cbf2/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf2
2442c19:dd:ec00:c25:c25e:dd:ddbb:cbf3/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf3
2452c19:dd:ec00:c25:c25e:dd:ddbb:cbf4/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf4
2462c19:dd:ec00:c25:c25e:dd:ddbb:cbf5/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf5
2472c19:dd:ec00:c25:c25e:dd:ddbb:cbf6/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf6
2482c19:dd:ec00:c25:c25e:dd:ddbb:cbf7/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf7
2492c19:dd:ec00:c25:c25e:dd:ddbb:cbf8/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf8
2502c19:dd:ec00:c25:c25e:dd:ddbb:cbf9/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbf9
2512c19:dd:ec00:c25:c25e:dd:ddbb:cbfa/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbfa
2522c19:dd:ec00:c25:c25e:dd:ddbb:cbfb/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbfb
2532c19:dd:ec00:c25:c25e:dd:ddbb:cbfc/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbfc
2542c19:dd:ec00:c25:c25e:dd:ddbb:cbfd/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbfd
2552c19:dd:ec00:c25:c25e:dd:ddbb:cbfe/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbfe
2562c19:dd:ec00:c25:c25e:dd:ddbb:cbff/1282c19:dd:ec00:c25:c25e:dd:ddbb:cbff

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.