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.

Network238.48.0.0/12
Usable hosts1,048,574
Subnet mask255.240.0.0
Addresses1,048,576
Network address
238.48.0.0
Broadcast address
238.63.255.255
First usable
238.48.0.1
Last usable
238.63.255.254
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
Multicast 224.0.0.0/4
One-to-many traffic, such as IPTV and routing protocols
Old class
D (multicast)
As numbers
Hex EE300000 · Integer 3996123136 to 3997171711
Reverse DNS zones
48.238.in-addr.arpa
49.238.in-addr.arpa
50.238.in-addr.arpa
51.238.in-addr.arpa
52.238.in-addr.arpa
53.238.in-addr.arpa
54.238.in-addr.arpa
55.238.in-addr.arpa
56.238.in-addr.arpa
57.238.in-addr.arpa
58.238.in-addr.arpa
59.238.in-addr.arpa
60.238.in-addr.arpa
61.238.in-addr.arpa
62.238.in-addr.arpa
63.238.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
Address11101110.00110000.00000000.00000000238.48.0.0
Subnet mask11111111.11110000.00000000.00000000255.240.0.0
Network11101110.00110000.00000000.00000000238.48.0.0
Broadcast11101110.00111111.11111111.11111111238.63.255.255
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)

238.48.0.0/12 makes 64 subnets of /18.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
1238.48.0.0/18238.48.0.1 to 238.48.63.254238.48.63.25516,382
2238.48.64.0/18238.48.64.1 to 238.48.127.254238.48.127.25516,382
3238.48.128.0/18238.48.128.1 to 238.48.191.254238.48.191.25516,382
4238.48.192.0/18238.48.192.1 to 238.48.255.254238.48.255.25516,382
5238.49.0.0/18238.49.0.1 to 238.49.63.254238.49.63.25516,382
6238.49.64.0/18238.49.64.1 to 238.49.127.254238.49.127.25516,382
7238.49.128.0/18238.49.128.1 to 238.49.191.254238.49.191.25516,382
8238.49.192.0/18238.49.192.1 to 238.49.255.254238.49.255.25516,382
9238.50.0.0/18238.50.0.1 to 238.50.63.254238.50.63.25516,382
10238.50.64.0/18238.50.64.1 to 238.50.127.254238.50.127.25516,382
11238.50.128.0/18238.50.128.1 to 238.50.191.254238.50.191.25516,382
12238.50.192.0/18238.50.192.1 to 238.50.255.254238.50.255.25516,382
13238.51.0.0/18238.51.0.1 to 238.51.63.254238.51.63.25516,382
14238.51.64.0/18238.51.64.1 to 238.51.127.254238.51.127.25516,382
15238.51.128.0/18238.51.128.1 to 238.51.191.254238.51.191.25516,382
16238.51.192.0/18238.51.192.1 to 238.51.255.254238.51.255.25516,382
17238.52.0.0/18238.52.0.1 to 238.52.63.254238.52.63.25516,382
18238.52.64.0/18238.52.64.1 to 238.52.127.254238.52.127.25516,382
19238.52.128.0/18238.52.128.1 to 238.52.191.254238.52.191.25516,382
20238.52.192.0/18238.52.192.1 to 238.52.255.254238.52.255.25516,382
21238.53.0.0/18238.53.0.1 to 238.53.63.254238.53.63.25516,382
22238.53.64.0/18238.53.64.1 to 238.53.127.254238.53.127.25516,382
23238.53.128.0/18238.53.128.1 to 238.53.191.254238.53.191.25516,382
24238.53.192.0/18238.53.192.1 to 238.53.255.254238.53.255.25516,382
25238.54.0.0/18238.54.0.1 to 238.54.63.254238.54.63.25516,382
26238.54.64.0/18238.54.64.1 to 238.54.127.254238.54.127.25516,382
27238.54.128.0/18238.54.128.1 to 238.54.191.254238.54.191.25516,382
28238.54.192.0/18238.54.192.1 to 238.54.255.254238.54.255.25516,382
29238.55.0.0/18238.55.0.1 to 238.55.63.254238.55.63.25516,382
30238.55.64.0/18238.55.64.1 to 238.55.127.254238.55.127.25516,382
31238.55.128.0/18238.55.128.1 to 238.55.191.254238.55.191.25516,382
32238.55.192.0/18238.55.192.1 to 238.55.255.254238.55.255.25516,382
33238.56.0.0/18238.56.0.1 to 238.56.63.254238.56.63.25516,382
34238.56.64.0/18238.56.64.1 to 238.56.127.254238.56.127.25516,382
35238.56.128.0/18238.56.128.1 to 238.56.191.254238.56.191.25516,382
36238.56.192.0/18238.56.192.1 to 238.56.255.254238.56.255.25516,382
37238.57.0.0/18238.57.0.1 to 238.57.63.254238.57.63.25516,382
38238.57.64.0/18238.57.64.1 to 238.57.127.254238.57.127.25516,382
39238.57.128.0/18238.57.128.1 to 238.57.191.254238.57.191.25516,382
40238.57.192.0/18238.57.192.1 to 238.57.255.254238.57.255.25516,382
41238.58.0.0/18238.58.0.1 to 238.58.63.254238.58.63.25516,382
42238.58.64.0/18238.58.64.1 to 238.58.127.254238.58.127.25516,382
43238.58.128.0/18238.58.128.1 to 238.58.191.254238.58.191.25516,382
44238.58.192.0/18238.58.192.1 to 238.58.255.254238.58.255.25516,382
45238.59.0.0/18238.59.0.1 to 238.59.63.254238.59.63.25516,382
46238.59.64.0/18238.59.64.1 to 238.59.127.254238.59.127.25516,382
47238.59.128.0/18238.59.128.1 to 238.59.191.254238.59.191.25516,382
48238.59.192.0/18238.59.192.1 to 238.59.255.254238.59.255.25516,382
49238.60.0.0/18238.60.0.1 to 238.60.63.254238.60.63.25516,382
50238.60.64.0/18238.60.64.1 to 238.60.127.254238.60.127.25516,382
51238.60.128.0/18238.60.128.1 to 238.60.191.254238.60.191.25516,382
52238.60.192.0/18238.60.192.1 to 238.60.255.254238.60.255.25516,382
53238.61.0.0/18238.61.0.1 to 238.61.63.254238.61.63.25516,382
54238.61.64.0/18238.61.64.1 to 238.61.127.254238.61.127.25516,382
55238.61.128.0/18238.61.128.1 to 238.61.191.254238.61.191.25516,382
56238.61.192.0/18238.61.192.1 to 238.61.255.254238.61.255.25516,382
57238.62.0.0/18238.62.0.1 to 238.62.63.254238.62.63.25516,382
58238.62.64.0/18238.62.64.1 to 238.62.127.254238.62.127.25516,382
59238.62.128.0/18238.62.128.1 to 238.62.191.254238.62.191.25516,382
60238.62.192.0/18238.62.192.1 to 238.62.255.254238.62.255.25516,382
61238.63.0.0/18238.63.0.1 to 238.63.63.254238.63.63.25516,382
62238.63.64.0/18238.63.64.1 to 238.63.127.254238.63.127.25516,382
63238.63.128.0/18238.63.128.1 to 238.63.191.254238.63.191.25516,382
64238.63.192.0/18238.63.192.1 to 238.63.255.254238.63.255.25516,382

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.