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.

Network243.0.0.0/8
Usable hosts16,777,214
Subnet mask255.0.0.0
Addresses16,777,216
Network address
243.0.0.0
Broadcast address
243.255.255.255
First usable
243.0.0.1
Last usable
243.255.255.254
Usable hosts
16,777,214 (16,777,216 less the network and broadcast addresses)
Subnet mask
255.0.0.0
Wildcard mask
0.255.255.255 (Cisco ACLs, OSPF)
Prefix length
/8
Type
Reserved 240.0.0.0/4
Future use; not usable on most systems
Old class
E (reserved)
As numbers
Hex F3000000 · Integer 4076863488 to 4093640703
Reverse DNS zone
243.in-addr.arpa
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address11110011..00000000.00000000.00000000243.0.0.0
Subnet mask11111111..00000000.00000000.00000000255.0.0.0
Network11110011..00000000.00000000.00000000243.0.0.0
Broadcast11110011..11111111.11111111.11111111243.255.255.255
Split into smaller subnets/15
/9 (2)/10 (4)/11 (8)/12 (16)/13 (32)/14 (64)/15 (128)/16 (256)/17 (512)/18 (1,024)/19 (2,048)/20 (4,096)

243.0.0.0/8 makes 128 subnets of /15.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
1243.0.0.0/15243.0.0.1 to 243.1.255.254243.1.255.255131,070
2243.2.0.0/15243.2.0.1 to 243.3.255.254243.3.255.255131,070
3243.4.0.0/15243.4.0.1 to 243.5.255.254243.5.255.255131,070
4243.6.0.0/15243.6.0.1 to 243.7.255.254243.7.255.255131,070
5243.8.0.0/15243.8.0.1 to 243.9.255.254243.9.255.255131,070
6243.10.0.0/15243.10.0.1 to 243.11.255.254243.11.255.255131,070
7243.12.0.0/15243.12.0.1 to 243.13.255.254243.13.255.255131,070
8243.14.0.0/15243.14.0.1 to 243.15.255.254243.15.255.255131,070
9243.16.0.0/15243.16.0.1 to 243.17.255.254243.17.255.255131,070
10243.18.0.0/15243.18.0.1 to 243.19.255.254243.19.255.255131,070
11243.20.0.0/15243.20.0.1 to 243.21.255.254243.21.255.255131,070
12243.22.0.0/15243.22.0.1 to 243.23.255.254243.23.255.255131,070
13243.24.0.0/15243.24.0.1 to 243.25.255.254243.25.255.255131,070
14243.26.0.0/15243.26.0.1 to 243.27.255.254243.27.255.255131,070
15243.28.0.0/15243.28.0.1 to 243.29.255.254243.29.255.255131,070
16243.30.0.0/15243.30.0.1 to 243.31.255.254243.31.255.255131,070
17243.32.0.0/15243.32.0.1 to 243.33.255.254243.33.255.255131,070
18243.34.0.0/15243.34.0.1 to 243.35.255.254243.35.255.255131,070
19243.36.0.0/15243.36.0.1 to 243.37.255.254243.37.255.255131,070
20243.38.0.0/15243.38.0.1 to 243.39.255.254243.39.255.255131,070
21243.40.0.0/15243.40.0.1 to 243.41.255.254243.41.255.255131,070
22243.42.0.0/15243.42.0.1 to 243.43.255.254243.43.255.255131,070
23243.44.0.0/15243.44.0.1 to 243.45.255.254243.45.255.255131,070
24243.46.0.0/15243.46.0.1 to 243.47.255.254243.47.255.255131,070
25243.48.0.0/15243.48.0.1 to 243.49.255.254243.49.255.255131,070
26243.50.0.0/15243.50.0.1 to 243.51.255.254243.51.255.255131,070
27243.52.0.0/15243.52.0.1 to 243.53.255.254243.53.255.255131,070
28243.54.0.0/15243.54.0.1 to 243.55.255.254243.55.255.255131,070
29243.56.0.0/15243.56.0.1 to 243.57.255.254243.57.255.255131,070
30243.58.0.0/15243.58.0.1 to 243.59.255.254243.59.255.255131,070
31243.60.0.0/15243.60.0.1 to 243.61.255.254243.61.255.255131,070
32243.62.0.0/15243.62.0.1 to 243.63.255.254243.63.255.255131,070
33243.64.0.0/15243.64.0.1 to 243.65.255.254243.65.255.255131,070
34243.66.0.0/15243.66.0.1 to 243.67.255.254243.67.255.255131,070
35243.68.0.0/15243.68.0.1 to 243.69.255.254243.69.255.255131,070
36243.70.0.0/15243.70.0.1 to 243.71.255.254243.71.255.255131,070
37243.72.0.0/15243.72.0.1 to 243.73.255.254243.73.255.255131,070
38243.74.0.0/15243.74.0.1 to 243.75.255.254243.75.255.255131,070
39243.76.0.0/15243.76.0.1 to 243.77.255.254243.77.255.255131,070
40243.78.0.0/15243.78.0.1 to 243.79.255.254243.79.255.255131,070
41243.80.0.0/15243.80.0.1 to 243.81.255.254243.81.255.255131,070
42243.82.0.0/15243.82.0.1 to 243.83.255.254243.83.255.255131,070
43243.84.0.0/15243.84.0.1 to 243.85.255.254243.85.255.255131,070
44243.86.0.0/15243.86.0.1 to 243.87.255.254243.87.255.255131,070
45243.88.0.0/15243.88.0.1 to 243.89.255.254243.89.255.255131,070
46243.90.0.0/15243.90.0.1 to 243.91.255.254243.91.255.255131,070
47243.92.0.0/15243.92.0.1 to 243.93.255.254243.93.255.255131,070
48243.94.0.0/15243.94.0.1 to 243.95.255.254243.95.255.255131,070
49243.96.0.0/15243.96.0.1 to 243.97.255.254243.97.255.255131,070
50243.98.0.0/15243.98.0.1 to 243.99.255.254243.99.255.255131,070
51243.100.0.0/15243.100.0.1 to 243.101.255.254243.101.255.255131,070
52243.102.0.0/15243.102.0.1 to 243.103.255.254243.103.255.255131,070
53243.104.0.0/15243.104.0.1 to 243.105.255.254243.105.255.255131,070
54243.106.0.0/15243.106.0.1 to 243.107.255.254243.107.255.255131,070
55243.108.0.0/15243.108.0.1 to 243.109.255.254243.109.255.255131,070
56243.110.0.0/15243.110.0.1 to 243.111.255.254243.111.255.255131,070
57243.112.0.0/15243.112.0.1 to 243.113.255.254243.113.255.255131,070
58243.114.0.0/15243.114.0.1 to 243.115.255.254243.115.255.255131,070
59243.116.0.0/15243.116.0.1 to 243.117.255.254243.117.255.255131,070
60243.118.0.0/15243.118.0.1 to 243.119.255.254243.119.255.255131,070
61243.120.0.0/15243.120.0.1 to 243.121.255.254243.121.255.255131,070
62243.122.0.0/15243.122.0.1 to 243.123.255.254243.123.255.255131,070
63243.124.0.0/15243.124.0.1 to 243.125.255.254243.125.255.255131,070
64243.126.0.0/15243.126.0.1 to 243.127.255.254243.127.255.255131,070
65243.128.0.0/15243.128.0.1 to 243.129.255.254243.129.255.255131,070
66243.130.0.0/15243.130.0.1 to 243.131.255.254243.131.255.255131,070
67243.132.0.0/15243.132.0.1 to 243.133.255.254243.133.255.255131,070
68243.134.0.0/15243.134.0.1 to 243.135.255.254243.135.255.255131,070
69243.136.0.0/15243.136.0.1 to 243.137.255.254243.137.255.255131,070
70243.138.0.0/15243.138.0.1 to 243.139.255.254243.139.255.255131,070
71243.140.0.0/15243.140.0.1 to 243.141.255.254243.141.255.255131,070
72243.142.0.0/15243.142.0.1 to 243.143.255.254243.143.255.255131,070
73243.144.0.0/15243.144.0.1 to 243.145.255.254243.145.255.255131,070
74243.146.0.0/15243.146.0.1 to 243.147.255.254243.147.255.255131,070
75243.148.0.0/15243.148.0.1 to 243.149.255.254243.149.255.255131,070
76243.150.0.0/15243.150.0.1 to 243.151.255.254243.151.255.255131,070
77243.152.0.0/15243.152.0.1 to 243.153.255.254243.153.255.255131,070
78243.154.0.0/15243.154.0.1 to 243.155.255.254243.155.255.255131,070
79243.156.0.0/15243.156.0.1 to 243.157.255.254243.157.255.255131,070
80243.158.0.0/15243.158.0.1 to 243.159.255.254243.159.255.255131,070
81243.160.0.0/15243.160.0.1 to 243.161.255.254243.161.255.255131,070
82243.162.0.0/15243.162.0.1 to 243.163.255.254243.163.255.255131,070
83243.164.0.0/15243.164.0.1 to 243.165.255.254243.165.255.255131,070
84243.166.0.0/15243.166.0.1 to 243.167.255.254243.167.255.255131,070
85243.168.0.0/15243.168.0.1 to 243.169.255.254243.169.255.255131,070
86243.170.0.0/15243.170.0.1 to 243.171.255.254243.171.255.255131,070
87243.172.0.0/15243.172.0.1 to 243.173.255.254243.173.255.255131,070
88243.174.0.0/15243.174.0.1 to 243.175.255.254243.175.255.255131,070
89243.176.0.0/15243.176.0.1 to 243.177.255.254243.177.255.255131,070
90243.178.0.0/15243.178.0.1 to 243.179.255.254243.179.255.255131,070
91243.180.0.0/15243.180.0.1 to 243.181.255.254243.181.255.255131,070
92243.182.0.0/15243.182.0.1 to 243.183.255.254243.183.255.255131,070
93243.184.0.0/15243.184.0.1 to 243.185.255.254243.185.255.255131,070
94243.186.0.0/15243.186.0.1 to 243.187.255.254243.187.255.255131,070
95243.188.0.0/15243.188.0.1 to 243.189.255.254243.189.255.255131,070
96243.190.0.0/15243.190.0.1 to 243.191.255.254243.191.255.255131,070
97243.192.0.0/15243.192.0.1 to 243.193.255.254243.193.255.255131,070
98243.194.0.0/15243.194.0.1 to 243.195.255.254243.195.255.255131,070
99243.196.0.0/15243.196.0.1 to 243.197.255.254243.197.255.255131,070
100243.198.0.0/15243.198.0.1 to 243.199.255.254243.199.255.255131,070
101243.200.0.0/15243.200.0.1 to 243.201.255.254243.201.255.255131,070
102243.202.0.0/15243.202.0.1 to 243.203.255.254243.203.255.255131,070
103243.204.0.0/15243.204.0.1 to 243.205.255.254243.205.255.255131,070
104243.206.0.0/15243.206.0.1 to 243.207.255.254243.207.255.255131,070
105243.208.0.0/15243.208.0.1 to 243.209.255.254243.209.255.255131,070
106243.210.0.0/15243.210.0.1 to 243.211.255.254243.211.255.255131,070
107243.212.0.0/15243.212.0.1 to 243.213.255.254243.213.255.255131,070
108243.214.0.0/15243.214.0.1 to 243.215.255.254243.215.255.255131,070
109243.216.0.0/15243.216.0.1 to 243.217.255.254243.217.255.255131,070
110243.218.0.0/15243.218.0.1 to 243.219.255.254243.219.255.255131,070
111243.220.0.0/15243.220.0.1 to 243.221.255.254243.221.255.255131,070
112243.222.0.0/15243.222.0.1 to 243.223.255.254243.223.255.255131,070
113243.224.0.0/15243.224.0.1 to 243.225.255.254243.225.255.255131,070
114243.226.0.0/15243.226.0.1 to 243.227.255.254243.227.255.255131,070
115243.228.0.0/15243.228.0.1 to 243.229.255.254243.229.255.255131,070
116243.230.0.0/15243.230.0.1 to 243.231.255.254243.231.255.255131,070
117243.232.0.0/15243.232.0.1 to 243.233.255.254243.233.255.255131,070
118243.234.0.0/15243.234.0.1 to 243.235.255.254243.235.255.255131,070
119243.236.0.0/15243.236.0.1 to 243.237.255.254243.237.255.255131,070
120243.238.0.0/15243.238.0.1 to 243.239.255.254243.239.255.255131,070
121243.240.0.0/15243.240.0.1 to 243.241.255.254243.241.255.255131,070
122243.242.0.0/15243.242.0.1 to 243.243.255.254243.243.255.255131,070
123243.244.0.0/15243.244.0.1 to 243.245.255.254243.245.255.255131,070
124243.246.0.0/15243.246.0.1 to 243.247.255.254243.247.255.255131,070
125243.248.0.0/15243.248.0.1 to 243.249.255.254243.249.255.255131,070
126243.250.0.0/15243.250.0.1 to 243.251.255.254243.251.255.255131,070
127243.252.0.0/15243.252.0.1 to 243.253.255.254243.253.255.255131,070
128243.254.0.0/15243.254.0.1 to 243.255.255.254243.255.255.255131,070

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.