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.

Network205.0.0.0/8
Usable hosts16,777,214
Subnet mask255.0.0.0
Addresses16,777,216
Network address
205.0.0.0
Broadcast address
205.255.255.255
First usable
205.0.0.1
Last usable
205.255.255.254
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
Public public
Old class
C (default /24)
As numbers
Hex CD000000 · Integer 3439329280 to 3456106495
Reverse DNS zone
205.in-addr.arpa
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address11001101..00000000.00000000.00000000205.0.0.0
Subnet mask11111111..00000000.00000000.00000000255.0.0.0
Network11001101..00000000.00000000.00000000205.0.0.0
Broadcast11001101..11111111.11111111.11111111205.255.255.255
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)

205.0.0.0/8 makes 128 subnets of /15.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
1205.0.0.0/15205.0.0.1 to 205.1.255.254205.1.255.255131,070
2205.2.0.0/15205.2.0.1 to 205.3.255.254205.3.255.255131,070
3205.4.0.0/15205.4.0.1 to 205.5.255.254205.5.255.255131,070
4205.6.0.0/15205.6.0.1 to 205.7.255.254205.7.255.255131,070
5205.8.0.0/15205.8.0.1 to 205.9.255.254205.9.255.255131,070
6205.10.0.0/15205.10.0.1 to 205.11.255.254205.11.255.255131,070
7205.12.0.0/15205.12.0.1 to 205.13.255.254205.13.255.255131,070
8205.14.0.0/15205.14.0.1 to 205.15.255.254205.15.255.255131,070
9205.16.0.0/15205.16.0.1 to 205.17.255.254205.17.255.255131,070
10205.18.0.0/15205.18.0.1 to 205.19.255.254205.19.255.255131,070
11205.20.0.0/15205.20.0.1 to 205.21.255.254205.21.255.255131,070
12205.22.0.0/15205.22.0.1 to 205.23.255.254205.23.255.255131,070
13205.24.0.0/15205.24.0.1 to 205.25.255.254205.25.255.255131,070
14205.26.0.0/15205.26.0.1 to 205.27.255.254205.27.255.255131,070
15205.28.0.0/15205.28.0.1 to 205.29.255.254205.29.255.255131,070
16205.30.0.0/15205.30.0.1 to 205.31.255.254205.31.255.255131,070
17205.32.0.0/15205.32.0.1 to 205.33.255.254205.33.255.255131,070
18205.34.0.0/15205.34.0.1 to 205.35.255.254205.35.255.255131,070
19205.36.0.0/15205.36.0.1 to 205.37.255.254205.37.255.255131,070
20205.38.0.0/15205.38.0.1 to 205.39.255.254205.39.255.255131,070
21205.40.0.0/15205.40.0.1 to 205.41.255.254205.41.255.255131,070
22205.42.0.0/15205.42.0.1 to 205.43.255.254205.43.255.255131,070
23205.44.0.0/15205.44.0.1 to 205.45.255.254205.45.255.255131,070
24205.46.0.0/15205.46.0.1 to 205.47.255.254205.47.255.255131,070
25205.48.0.0/15205.48.0.1 to 205.49.255.254205.49.255.255131,070
26205.50.0.0/15205.50.0.1 to 205.51.255.254205.51.255.255131,070
27205.52.0.0/15205.52.0.1 to 205.53.255.254205.53.255.255131,070
28205.54.0.0/15205.54.0.1 to 205.55.255.254205.55.255.255131,070
29205.56.0.0/15205.56.0.1 to 205.57.255.254205.57.255.255131,070
30205.58.0.0/15205.58.0.1 to 205.59.255.254205.59.255.255131,070
31205.60.0.0/15205.60.0.1 to 205.61.255.254205.61.255.255131,070
32205.62.0.0/15205.62.0.1 to 205.63.255.254205.63.255.255131,070
33205.64.0.0/15205.64.0.1 to 205.65.255.254205.65.255.255131,070
34205.66.0.0/15205.66.0.1 to 205.67.255.254205.67.255.255131,070
35205.68.0.0/15205.68.0.1 to 205.69.255.254205.69.255.255131,070
36205.70.0.0/15205.70.0.1 to 205.71.255.254205.71.255.255131,070
37205.72.0.0/15205.72.0.1 to 205.73.255.254205.73.255.255131,070
38205.74.0.0/15205.74.0.1 to 205.75.255.254205.75.255.255131,070
39205.76.0.0/15205.76.0.1 to 205.77.255.254205.77.255.255131,070
40205.78.0.0/15205.78.0.1 to 205.79.255.254205.79.255.255131,070
41205.80.0.0/15205.80.0.1 to 205.81.255.254205.81.255.255131,070
42205.82.0.0/15205.82.0.1 to 205.83.255.254205.83.255.255131,070
43205.84.0.0/15205.84.0.1 to 205.85.255.254205.85.255.255131,070
44205.86.0.0/15205.86.0.1 to 205.87.255.254205.87.255.255131,070
45205.88.0.0/15205.88.0.1 to 205.89.255.254205.89.255.255131,070
46205.90.0.0/15205.90.0.1 to 205.91.255.254205.91.255.255131,070
47205.92.0.0/15205.92.0.1 to 205.93.255.254205.93.255.255131,070
48205.94.0.0/15205.94.0.1 to 205.95.255.254205.95.255.255131,070
49205.96.0.0/15205.96.0.1 to 205.97.255.254205.97.255.255131,070
50205.98.0.0/15205.98.0.1 to 205.99.255.254205.99.255.255131,070
51205.100.0.0/15205.100.0.1 to 205.101.255.254205.101.255.255131,070
52205.102.0.0/15205.102.0.1 to 205.103.255.254205.103.255.255131,070
53205.104.0.0/15205.104.0.1 to 205.105.255.254205.105.255.255131,070
54205.106.0.0/15205.106.0.1 to 205.107.255.254205.107.255.255131,070
55205.108.0.0/15205.108.0.1 to 205.109.255.254205.109.255.255131,070
56205.110.0.0/15205.110.0.1 to 205.111.255.254205.111.255.255131,070
57205.112.0.0/15205.112.0.1 to 205.113.255.254205.113.255.255131,070
58205.114.0.0/15205.114.0.1 to 205.115.255.254205.115.255.255131,070
59205.116.0.0/15205.116.0.1 to 205.117.255.254205.117.255.255131,070
60205.118.0.0/15205.118.0.1 to 205.119.255.254205.119.255.255131,070
61205.120.0.0/15205.120.0.1 to 205.121.255.254205.121.255.255131,070
62205.122.0.0/15205.122.0.1 to 205.123.255.254205.123.255.255131,070
63205.124.0.0/15205.124.0.1 to 205.125.255.254205.125.255.255131,070
64205.126.0.0/15205.126.0.1 to 205.127.255.254205.127.255.255131,070
65205.128.0.0/15205.128.0.1 to 205.129.255.254205.129.255.255131,070
66205.130.0.0/15205.130.0.1 to 205.131.255.254205.131.255.255131,070
67205.132.0.0/15205.132.0.1 to 205.133.255.254205.133.255.255131,070
68205.134.0.0/15205.134.0.1 to 205.135.255.254205.135.255.255131,070
69205.136.0.0/15205.136.0.1 to 205.137.255.254205.137.255.255131,070
70205.138.0.0/15205.138.0.1 to 205.139.255.254205.139.255.255131,070
71205.140.0.0/15205.140.0.1 to 205.141.255.254205.141.255.255131,070
72205.142.0.0/15205.142.0.1 to 205.143.255.254205.143.255.255131,070
73205.144.0.0/15205.144.0.1 to 205.145.255.254205.145.255.255131,070
74205.146.0.0/15205.146.0.1 to 205.147.255.254205.147.255.255131,070
75205.148.0.0/15205.148.0.1 to 205.149.255.254205.149.255.255131,070
76205.150.0.0/15205.150.0.1 to 205.151.255.254205.151.255.255131,070
77205.152.0.0/15205.152.0.1 to 205.153.255.254205.153.255.255131,070
78205.154.0.0/15205.154.0.1 to 205.155.255.254205.155.255.255131,070
79205.156.0.0/15205.156.0.1 to 205.157.255.254205.157.255.255131,070
80205.158.0.0/15205.158.0.1 to 205.159.255.254205.159.255.255131,070
81205.160.0.0/15205.160.0.1 to 205.161.255.254205.161.255.255131,070
82205.162.0.0/15205.162.0.1 to 205.163.255.254205.163.255.255131,070
83205.164.0.0/15205.164.0.1 to 205.165.255.254205.165.255.255131,070
84205.166.0.0/15205.166.0.1 to 205.167.255.254205.167.255.255131,070
85205.168.0.0/15205.168.0.1 to 205.169.255.254205.169.255.255131,070
86205.170.0.0/15205.170.0.1 to 205.171.255.254205.171.255.255131,070
87205.172.0.0/15205.172.0.1 to 205.173.255.254205.173.255.255131,070
88205.174.0.0/15205.174.0.1 to 205.175.255.254205.175.255.255131,070
89205.176.0.0/15205.176.0.1 to 205.177.255.254205.177.255.255131,070
90205.178.0.0/15205.178.0.1 to 205.179.255.254205.179.255.255131,070
91205.180.0.0/15205.180.0.1 to 205.181.255.254205.181.255.255131,070
92205.182.0.0/15205.182.0.1 to 205.183.255.254205.183.255.255131,070
93205.184.0.0/15205.184.0.1 to 205.185.255.254205.185.255.255131,070
94205.186.0.0/15205.186.0.1 to 205.187.255.254205.187.255.255131,070
95205.188.0.0/15205.188.0.1 to 205.189.255.254205.189.255.255131,070
96205.190.0.0/15205.190.0.1 to 205.191.255.254205.191.255.255131,070
97205.192.0.0/15205.192.0.1 to 205.193.255.254205.193.255.255131,070
98205.194.0.0/15205.194.0.1 to 205.195.255.254205.195.255.255131,070
99205.196.0.0/15205.196.0.1 to 205.197.255.254205.197.255.255131,070
100205.198.0.0/15205.198.0.1 to 205.199.255.254205.199.255.255131,070
101205.200.0.0/15205.200.0.1 to 205.201.255.254205.201.255.255131,070
102205.202.0.0/15205.202.0.1 to 205.203.255.254205.203.255.255131,070
103205.204.0.0/15205.204.0.1 to 205.205.255.254205.205.255.255131,070
104205.206.0.0/15205.206.0.1 to 205.207.255.254205.207.255.255131,070
105205.208.0.0/15205.208.0.1 to 205.209.255.254205.209.255.255131,070
106205.210.0.0/15205.210.0.1 to 205.211.255.254205.211.255.255131,070
107205.212.0.0/15205.212.0.1 to 205.213.255.254205.213.255.255131,070
108205.214.0.0/15205.214.0.1 to 205.215.255.254205.215.255.255131,070
109205.216.0.0/15205.216.0.1 to 205.217.255.254205.217.255.255131,070
110205.218.0.0/15205.218.0.1 to 205.219.255.254205.219.255.255131,070
111205.220.0.0/15205.220.0.1 to 205.221.255.254205.221.255.255131,070
112205.222.0.0/15205.222.0.1 to 205.223.255.254205.223.255.255131,070
113205.224.0.0/15205.224.0.1 to 205.225.255.254205.225.255.255131,070
114205.226.0.0/15205.226.0.1 to 205.227.255.254205.227.255.255131,070
115205.228.0.0/15205.228.0.1 to 205.229.255.254205.229.255.255131,070
116205.230.0.0/15205.230.0.1 to 205.231.255.254205.231.255.255131,070
117205.232.0.0/15205.232.0.1 to 205.233.255.254205.233.255.255131,070
118205.234.0.0/15205.234.0.1 to 205.235.255.254205.235.255.255131,070
119205.236.0.0/15205.236.0.1 to 205.237.255.254205.237.255.255131,070
120205.238.0.0/15205.238.0.1 to 205.239.255.254205.239.255.255131,070
121205.240.0.0/15205.240.0.1 to 205.241.255.254205.241.255.255131,070
122205.242.0.0/15205.242.0.1 to 205.243.255.254205.243.255.255131,070
123205.244.0.0/15205.244.0.1 to 205.245.255.254205.245.255.255131,070
124205.246.0.0/15205.246.0.1 to 205.247.255.254205.247.255.255131,070
125205.248.0.0/15205.248.0.1 to 205.249.255.254205.249.255.255131,070
126205.250.0.0/15205.250.0.1 to 205.251.255.254205.251.255.255131,070
127205.252.0.0/15205.252.0.1 to 205.253.255.254205.253.255.255131,070
128205.254.0.0/15205.254.0.1 to 205.255.255.254205.255.255.255131,070

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.