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.

Network237.0.0.0/8
Usable hosts16,777,214
Subnet mask255.0.0.0
Addresses16,777,216
Network address
237.0.0.0
Broadcast address
237.255.255.255
First usable
237.0.0.1
Last usable
237.255.255.254
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
Multicast 224.0.0.0/4
One-to-many traffic, such as IPTV and routing protocols
Old class
D (multicast)
As numbers
Hex ED000000 · Integer 3976200192 to 3992977407
Reverse DNS zone
237.in-addr.arpa
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address11101101..00000000.00000000.00000000237.0.0.0
Subnet mask11111111..00000000.00000000.00000000255.0.0.0
Network11101101..00000000.00000000.00000000237.0.0.0
Broadcast11101101..11111111.11111111.11111111237.255.255.255
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)

237.0.0.0/8 makes 128 subnets of /15.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
1237.0.0.0/15237.0.0.1 to 237.1.255.254237.1.255.255131,070
2237.2.0.0/15237.2.0.1 to 237.3.255.254237.3.255.255131,070
3237.4.0.0/15237.4.0.1 to 237.5.255.254237.5.255.255131,070
4237.6.0.0/15237.6.0.1 to 237.7.255.254237.7.255.255131,070
5237.8.0.0/15237.8.0.1 to 237.9.255.254237.9.255.255131,070
6237.10.0.0/15237.10.0.1 to 237.11.255.254237.11.255.255131,070
7237.12.0.0/15237.12.0.1 to 237.13.255.254237.13.255.255131,070
8237.14.0.0/15237.14.0.1 to 237.15.255.254237.15.255.255131,070
9237.16.0.0/15237.16.0.1 to 237.17.255.254237.17.255.255131,070
10237.18.0.0/15237.18.0.1 to 237.19.255.254237.19.255.255131,070
11237.20.0.0/15237.20.0.1 to 237.21.255.254237.21.255.255131,070
12237.22.0.0/15237.22.0.1 to 237.23.255.254237.23.255.255131,070
13237.24.0.0/15237.24.0.1 to 237.25.255.254237.25.255.255131,070
14237.26.0.0/15237.26.0.1 to 237.27.255.254237.27.255.255131,070
15237.28.0.0/15237.28.0.1 to 237.29.255.254237.29.255.255131,070
16237.30.0.0/15237.30.0.1 to 237.31.255.254237.31.255.255131,070
17237.32.0.0/15237.32.0.1 to 237.33.255.254237.33.255.255131,070
18237.34.0.0/15237.34.0.1 to 237.35.255.254237.35.255.255131,070
19237.36.0.0/15237.36.0.1 to 237.37.255.254237.37.255.255131,070
20237.38.0.0/15237.38.0.1 to 237.39.255.254237.39.255.255131,070
21237.40.0.0/15237.40.0.1 to 237.41.255.254237.41.255.255131,070
22237.42.0.0/15237.42.0.1 to 237.43.255.254237.43.255.255131,070
23237.44.0.0/15237.44.0.1 to 237.45.255.254237.45.255.255131,070
24237.46.0.0/15237.46.0.1 to 237.47.255.254237.47.255.255131,070
25237.48.0.0/15237.48.0.1 to 237.49.255.254237.49.255.255131,070
26237.50.0.0/15237.50.0.1 to 237.51.255.254237.51.255.255131,070
27237.52.0.0/15237.52.0.1 to 237.53.255.254237.53.255.255131,070
28237.54.0.0/15237.54.0.1 to 237.55.255.254237.55.255.255131,070
29237.56.0.0/15237.56.0.1 to 237.57.255.254237.57.255.255131,070
30237.58.0.0/15237.58.0.1 to 237.59.255.254237.59.255.255131,070
31237.60.0.0/15237.60.0.1 to 237.61.255.254237.61.255.255131,070
32237.62.0.0/15237.62.0.1 to 237.63.255.254237.63.255.255131,070
33237.64.0.0/15237.64.0.1 to 237.65.255.254237.65.255.255131,070
34237.66.0.0/15237.66.0.1 to 237.67.255.254237.67.255.255131,070
35237.68.0.0/15237.68.0.1 to 237.69.255.254237.69.255.255131,070
36237.70.0.0/15237.70.0.1 to 237.71.255.254237.71.255.255131,070
37237.72.0.0/15237.72.0.1 to 237.73.255.254237.73.255.255131,070
38237.74.0.0/15237.74.0.1 to 237.75.255.254237.75.255.255131,070
39237.76.0.0/15237.76.0.1 to 237.77.255.254237.77.255.255131,070
40237.78.0.0/15237.78.0.1 to 237.79.255.254237.79.255.255131,070
41237.80.0.0/15237.80.0.1 to 237.81.255.254237.81.255.255131,070
42237.82.0.0/15237.82.0.1 to 237.83.255.254237.83.255.255131,070
43237.84.0.0/15237.84.0.1 to 237.85.255.254237.85.255.255131,070
44237.86.0.0/15237.86.0.1 to 237.87.255.254237.87.255.255131,070
45237.88.0.0/15237.88.0.1 to 237.89.255.254237.89.255.255131,070
46237.90.0.0/15237.90.0.1 to 237.91.255.254237.91.255.255131,070
47237.92.0.0/15237.92.0.1 to 237.93.255.254237.93.255.255131,070
48237.94.0.0/15237.94.0.1 to 237.95.255.254237.95.255.255131,070
49237.96.0.0/15237.96.0.1 to 237.97.255.254237.97.255.255131,070
50237.98.0.0/15237.98.0.1 to 237.99.255.254237.99.255.255131,070
51237.100.0.0/15237.100.0.1 to 237.101.255.254237.101.255.255131,070
52237.102.0.0/15237.102.0.1 to 237.103.255.254237.103.255.255131,070
53237.104.0.0/15237.104.0.1 to 237.105.255.254237.105.255.255131,070
54237.106.0.0/15237.106.0.1 to 237.107.255.254237.107.255.255131,070
55237.108.0.0/15237.108.0.1 to 237.109.255.254237.109.255.255131,070
56237.110.0.0/15237.110.0.1 to 237.111.255.254237.111.255.255131,070
57237.112.0.0/15237.112.0.1 to 237.113.255.254237.113.255.255131,070
58237.114.0.0/15237.114.0.1 to 237.115.255.254237.115.255.255131,070
59237.116.0.0/15237.116.0.1 to 237.117.255.254237.117.255.255131,070
60237.118.0.0/15237.118.0.1 to 237.119.255.254237.119.255.255131,070
61237.120.0.0/15237.120.0.1 to 237.121.255.254237.121.255.255131,070
62237.122.0.0/15237.122.0.1 to 237.123.255.254237.123.255.255131,070
63237.124.0.0/15237.124.0.1 to 237.125.255.254237.125.255.255131,070
64237.126.0.0/15237.126.0.1 to 237.127.255.254237.127.255.255131,070
65237.128.0.0/15237.128.0.1 to 237.129.255.254237.129.255.255131,070
66237.130.0.0/15237.130.0.1 to 237.131.255.254237.131.255.255131,070
67237.132.0.0/15237.132.0.1 to 237.133.255.254237.133.255.255131,070
68237.134.0.0/15237.134.0.1 to 237.135.255.254237.135.255.255131,070
69237.136.0.0/15237.136.0.1 to 237.137.255.254237.137.255.255131,070
70237.138.0.0/15237.138.0.1 to 237.139.255.254237.139.255.255131,070
71237.140.0.0/15237.140.0.1 to 237.141.255.254237.141.255.255131,070
72237.142.0.0/15237.142.0.1 to 237.143.255.254237.143.255.255131,070
73237.144.0.0/15237.144.0.1 to 237.145.255.254237.145.255.255131,070
74237.146.0.0/15237.146.0.1 to 237.147.255.254237.147.255.255131,070
75237.148.0.0/15237.148.0.1 to 237.149.255.254237.149.255.255131,070
76237.150.0.0/15237.150.0.1 to 237.151.255.254237.151.255.255131,070
77237.152.0.0/15237.152.0.1 to 237.153.255.254237.153.255.255131,070
78237.154.0.0/15237.154.0.1 to 237.155.255.254237.155.255.255131,070
79237.156.0.0/15237.156.0.1 to 237.157.255.254237.157.255.255131,070
80237.158.0.0/15237.158.0.1 to 237.159.255.254237.159.255.255131,070
81237.160.0.0/15237.160.0.1 to 237.161.255.254237.161.255.255131,070
82237.162.0.0/15237.162.0.1 to 237.163.255.254237.163.255.255131,070
83237.164.0.0/15237.164.0.1 to 237.165.255.254237.165.255.255131,070
84237.166.0.0/15237.166.0.1 to 237.167.255.254237.167.255.255131,070
85237.168.0.0/15237.168.0.1 to 237.169.255.254237.169.255.255131,070
86237.170.0.0/15237.170.0.1 to 237.171.255.254237.171.255.255131,070
87237.172.0.0/15237.172.0.1 to 237.173.255.254237.173.255.255131,070
88237.174.0.0/15237.174.0.1 to 237.175.255.254237.175.255.255131,070
89237.176.0.0/15237.176.0.1 to 237.177.255.254237.177.255.255131,070
90237.178.0.0/15237.178.0.1 to 237.179.255.254237.179.255.255131,070
91237.180.0.0/15237.180.0.1 to 237.181.255.254237.181.255.255131,070
92237.182.0.0/15237.182.0.1 to 237.183.255.254237.183.255.255131,070
93237.184.0.0/15237.184.0.1 to 237.185.255.254237.185.255.255131,070
94237.186.0.0/15237.186.0.1 to 237.187.255.254237.187.255.255131,070
95237.188.0.0/15237.188.0.1 to 237.189.255.254237.189.255.255131,070
96237.190.0.0/15237.190.0.1 to 237.191.255.254237.191.255.255131,070
97237.192.0.0/15237.192.0.1 to 237.193.255.254237.193.255.255131,070
98237.194.0.0/15237.194.0.1 to 237.195.255.254237.195.255.255131,070
99237.196.0.0/15237.196.0.1 to 237.197.255.254237.197.255.255131,070
100237.198.0.0/15237.198.0.1 to 237.199.255.254237.199.255.255131,070
101237.200.0.0/15237.200.0.1 to 237.201.255.254237.201.255.255131,070
102237.202.0.0/15237.202.0.1 to 237.203.255.254237.203.255.255131,070
103237.204.0.0/15237.204.0.1 to 237.205.255.254237.205.255.255131,070
104237.206.0.0/15237.206.0.1 to 237.207.255.254237.207.255.255131,070
105237.208.0.0/15237.208.0.1 to 237.209.255.254237.209.255.255131,070
106237.210.0.0/15237.210.0.1 to 237.211.255.254237.211.255.255131,070
107237.212.0.0/15237.212.0.1 to 237.213.255.254237.213.255.255131,070
108237.214.0.0/15237.214.0.1 to 237.215.255.254237.215.255.255131,070
109237.216.0.0/15237.216.0.1 to 237.217.255.254237.217.255.255131,070
110237.218.0.0/15237.218.0.1 to 237.219.255.254237.219.255.255131,070
111237.220.0.0/15237.220.0.1 to 237.221.255.254237.221.255.255131,070
112237.222.0.0/15237.222.0.1 to 237.223.255.254237.223.255.255131,070
113237.224.0.0/15237.224.0.1 to 237.225.255.254237.225.255.255131,070
114237.226.0.0/15237.226.0.1 to 237.227.255.254237.227.255.255131,070
115237.228.0.0/15237.228.0.1 to 237.229.255.254237.229.255.255131,070
116237.230.0.0/15237.230.0.1 to 237.231.255.254237.231.255.255131,070
117237.232.0.0/15237.232.0.1 to 237.233.255.254237.233.255.255131,070
118237.234.0.0/15237.234.0.1 to 237.235.255.254237.235.255.255131,070
119237.236.0.0/15237.236.0.1 to 237.237.255.254237.237.255.255131,070
120237.238.0.0/15237.238.0.1 to 237.239.255.254237.239.255.255131,070
121237.240.0.0/15237.240.0.1 to 237.241.255.254237.241.255.255131,070
122237.242.0.0/15237.242.0.1 to 237.243.255.254237.243.255.255131,070
123237.244.0.0/15237.244.0.1 to 237.245.255.254237.245.255.255131,070
124237.246.0.0/15237.246.0.1 to 237.247.255.254237.247.255.255131,070
125237.248.0.0/15237.248.0.1 to 237.249.255.254237.249.255.255131,070
126237.250.0.0/15237.250.0.1 to 237.251.255.254237.251.255.255131,070
127237.252.0.0/15237.252.0.1 to 237.253.255.254237.253.255.255131,070
128237.254.0.0/15237.254.0.1 to 237.255.255.254237.255.255.255131,070

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.