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.

Network252.64.0.0/12
Usable hosts1,048,574
Subnet mask255.240.0.0
Addresses1,048,576
Network address
252.64.0.0
Broadcast address
252.79.255.255
First usable
252.64.0.1
Last usable
252.79.255.254
Usable hosts
1,048,574 (1,048,576 less the network and broadcast addresses)
Subnet mask
255.240.0.0
Wildcard mask
0.15.255.255 (Cisco ACLs, OSPF)
Prefix length
/12
Type
Reserved 240.0.0.0/4
Future use; not usable on most systems
Old class
E (reserved)
As numbers
Hex FC400000 · Integer 4232052736 to 4233101311
Reverse DNS zones
64.252.in-addr.arpa
65.252.in-addr.arpa
66.252.in-addr.arpa
67.252.in-addr.arpa
68.252.in-addr.arpa
69.252.in-addr.arpa
70.252.in-addr.arpa
71.252.in-addr.arpa
72.252.in-addr.arpa
73.252.in-addr.arpa
74.252.in-addr.arpa
75.252.in-addr.arpa
76.252.in-addr.arpa
77.252.in-addr.arpa
78.252.in-addr.arpa
79.252.in-addr.arpa
/12 is not on a 8-bit boundary, so it takes 16 /16 zones.
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address11111100.01000000.00000000.00000000252.64.0.0
Subnet mask11111111.11110000.00000000.00000000255.240.0.0
Network11111100.01000000.00000000.00000000252.64.0.0
Broadcast11111100.01001111.11111111.11111111252.79.255.255
Split into smaller subnets/18
/13 (2)/14 (4)/15 (8)/16 (16)/17 (32)/18 (64)/19 (128)/20 (256)/21 (512)/22 (1,024)/23 (2,048)/24 (4,096)

252.64.0.0/12 makes 64 subnets of /18.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
1252.64.0.0/18252.64.0.1 to 252.64.63.254252.64.63.25516,382
2252.64.64.0/18252.64.64.1 to 252.64.127.254252.64.127.25516,382
3252.64.128.0/18252.64.128.1 to 252.64.191.254252.64.191.25516,382
4252.64.192.0/18252.64.192.1 to 252.64.255.254252.64.255.25516,382
5252.65.0.0/18252.65.0.1 to 252.65.63.254252.65.63.25516,382
6252.65.64.0/18252.65.64.1 to 252.65.127.254252.65.127.25516,382
7252.65.128.0/18252.65.128.1 to 252.65.191.254252.65.191.25516,382
8252.65.192.0/18252.65.192.1 to 252.65.255.254252.65.255.25516,382
9252.66.0.0/18252.66.0.1 to 252.66.63.254252.66.63.25516,382
10252.66.64.0/18252.66.64.1 to 252.66.127.254252.66.127.25516,382
11252.66.128.0/18252.66.128.1 to 252.66.191.254252.66.191.25516,382
12252.66.192.0/18252.66.192.1 to 252.66.255.254252.66.255.25516,382
13252.67.0.0/18252.67.0.1 to 252.67.63.254252.67.63.25516,382
14252.67.64.0/18252.67.64.1 to 252.67.127.254252.67.127.25516,382
15252.67.128.0/18252.67.128.1 to 252.67.191.254252.67.191.25516,382
16252.67.192.0/18252.67.192.1 to 252.67.255.254252.67.255.25516,382
17252.68.0.0/18252.68.0.1 to 252.68.63.254252.68.63.25516,382
18252.68.64.0/18252.68.64.1 to 252.68.127.254252.68.127.25516,382
19252.68.128.0/18252.68.128.1 to 252.68.191.254252.68.191.25516,382
20252.68.192.0/18252.68.192.1 to 252.68.255.254252.68.255.25516,382
21252.69.0.0/18252.69.0.1 to 252.69.63.254252.69.63.25516,382
22252.69.64.0/18252.69.64.1 to 252.69.127.254252.69.127.25516,382
23252.69.128.0/18252.69.128.1 to 252.69.191.254252.69.191.25516,382
24252.69.192.0/18252.69.192.1 to 252.69.255.254252.69.255.25516,382
25252.70.0.0/18252.70.0.1 to 252.70.63.254252.70.63.25516,382
26252.70.64.0/18252.70.64.1 to 252.70.127.254252.70.127.25516,382
27252.70.128.0/18252.70.128.1 to 252.70.191.254252.70.191.25516,382
28252.70.192.0/18252.70.192.1 to 252.70.255.254252.70.255.25516,382
29252.71.0.0/18252.71.0.1 to 252.71.63.254252.71.63.25516,382
30252.71.64.0/18252.71.64.1 to 252.71.127.254252.71.127.25516,382
31252.71.128.0/18252.71.128.1 to 252.71.191.254252.71.191.25516,382
32252.71.192.0/18252.71.192.1 to 252.71.255.254252.71.255.25516,382
33252.72.0.0/18252.72.0.1 to 252.72.63.254252.72.63.25516,382
34252.72.64.0/18252.72.64.1 to 252.72.127.254252.72.127.25516,382
35252.72.128.0/18252.72.128.1 to 252.72.191.254252.72.191.25516,382
36252.72.192.0/18252.72.192.1 to 252.72.255.254252.72.255.25516,382
37252.73.0.0/18252.73.0.1 to 252.73.63.254252.73.63.25516,382
38252.73.64.0/18252.73.64.1 to 252.73.127.254252.73.127.25516,382
39252.73.128.0/18252.73.128.1 to 252.73.191.254252.73.191.25516,382
40252.73.192.0/18252.73.192.1 to 252.73.255.254252.73.255.25516,382
41252.74.0.0/18252.74.0.1 to 252.74.63.254252.74.63.25516,382
42252.74.64.0/18252.74.64.1 to 252.74.127.254252.74.127.25516,382
43252.74.128.0/18252.74.128.1 to 252.74.191.254252.74.191.25516,382
44252.74.192.0/18252.74.192.1 to 252.74.255.254252.74.255.25516,382
45252.75.0.0/18252.75.0.1 to 252.75.63.254252.75.63.25516,382
46252.75.64.0/18252.75.64.1 to 252.75.127.254252.75.127.25516,382
47252.75.128.0/18252.75.128.1 to 252.75.191.254252.75.191.25516,382
48252.75.192.0/18252.75.192.1 to 252.75.255.254252.75.255.25516,382
49252.76.0.0/18252.76.0.1 to 252.76.63.254252.76.63.25516,382
50252.76.64.0/18252.76.64.1 to 252.76.127.254252.76.127.25516,382
51252.76.128.0/18252.76.128.1 to 252.76.191.254252.76.191.25516,382
52252.76.192.0/18252.76.192.1 to 252.76.255.254252.76.255.25516,382
53252.77.0.0/18252.77.0.1 to 252.77.63.254252.77.63.25516,382
54252.77.64.0/18252.77.64.1 to 252.77.127.254252.77.127.25516,382
55252.77.128.0/18252.77.128.1 to 252.77.191.254252.77.191.25516,382
56252.77.192.0/18252.77.192.1 to 252.77.255.254252.77.255.25516,382
57252.78.0.0/18252.78.0.1 to 252.78.63.254252.78.63.25516,382
58252.78.64.0/18252.78.64.1 to 252.78.127.254252.78.127.25516,382
59252.78.128.0/18252.78.128.1 to 252.78.191.254252.78.191.25516,382
60252.78.192.0/18252.78.192.1 to 252.78.255.254252.78.255.25516,382
61252.79.0.0/18252.79.0.1 to 252.79.63.254252.79.63.25516,382
62252.79.64.0/18252.79.64.1 to 252.79.127.254252.79.127.25516,382
63252.79.128.0/18252.79.128.1 to 252.79.191.254252.79.191.25516,382
64252.79.192.0/18252.79.192.1 to 252.79.255.254252.79.255.25516,382

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.