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.

Network204.0.0.0/6
Usable hosts67,108,862
Subnet mask252.0.0.0
Addresses67,108,864
Network address
204.0.0.0
Broadcast address
207.255.255.255
First usable
204.0.0.1
Last usable
207.255.255.254
Usable hosts
67,108,862 (67,108,864 less the network and broadcast addresses)
Subnet mask
252.0.0.0
Wildcard mask
3.255.255.255 (Cisco ACLs, OSPF)
Prefix length
/6
Type
Public public
Old class
C (default /24)
As numbers
Hex CC000000 · Integer 3422552064 to 3489660927
Reverse DNS zones
204.in-addr.arpa
205.in-addr.arpa
206.in-addr.arpa
207.in-addr.arpa
/6 is not on a 8-bit boundary, so it takes 4 /8 zones.
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address11001100.00000000.00000000.00000000204.0.0.0
Subnet mask11111100.00000000.00000000.00000000252.0.0.0
Network11001100.00000000.00000000.00000000204.0.0.0
Broadcast11001111.11111111.11111111.11111111207.255.255.255
Split into smaller subnets/13
/7 (2)/8 (4)/9 (8)/10 (16)/11 (32)/12 (64)/13 (128)/14 (256)/15 (512)/16 (1,024)/17 (2,048)/18 (4,096)

204.0.0.0/6 makes 128 subnets of /13.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
1204.0.0.0/13204.0.0.1 to 204.7.255.254204.7.255.255524,286
2204.8.0.0/13204.8.0.1 to 204.15.255.254204.15.255.255524,286
3204.16.0.0/13204.16.0.1 to 204.23.255.254204.23.255.255524,286
4204.24.0.0/13204.24.0.1 to 204.31.255.254204.31.255.255524,286
5204.32.0.0/13204.32.0.1 to 204.39.255.254204.39.255.255524,286
6204.40.0.0/13204.40.0.1 to 204.47.255.254204.47.255.255524,286
7204.48.0.0/13204.48.0.1 to 204.55.255.254204.55.255.255524,286
8204.56.0.0/13204.56.0.1 to 204.63.255.254204.63.255.255524,286
9204.64.0.0/13204.64.0.1 to 204.71.255.254204.71.255.255524,286
10204.72.0.0/13204.72.0.1 to 204.79.255.254204.79.255.255524,286
11204.80.0.0/13204.80.0.1 to 204.87.255.254204.87.255.255524,286
12204.88.0.0/13204.88.0.1 to 204.95.255.254204.95.255.255524,286
13204.96.0.0/13204.96.0.1 to 204.103.255.254204.103.255.255524,286
14204.104.0.0/13204.104.0.1 to 204.111.255.254204.111.255.255524,286
15204.112.0.0/13204.112.0.1 to 204.119.255.254204.119.255.255524,286
16204.120.0.0/13204.120.0.1 to 204.127.255.254204.127.255.255524,286
17204.128.0.0/13204.128.0.1 to 204.135.255.254204.135.255.255524,286
18204.136.0.0/13204.136.0.1 to 204.143.255.254204.143.255.255524,286
19204.144.0.0/13204.144.0.1 to 204.151.255.254204.151.255.255524,286
20204.152.0.0/13204.152.0.1 to 204.159.255.254204.159.255.255524,286
21204.160.0.0/13204.160.0.1 to 204.167.255.254204.167.255.255524,286
22204.168.0.0/13204.168.0.1 to 204.175.255.254204.175.255.255524,286
23204.176.0.0/13204.176.0.1 to 204.183.255.254204.183.255.255524,286
24204.184.0.0/13204.184.0.1 to 204.191.255.254204.191.255.255524,286
25204.192.0.0/13204.192.0.1 to 204.199.255.254204.199.255.255524,286
26204.200.0.0/13204.200.0.1 to 204.207.255.254204.207.255.255524,286
27204.208.0.0/13204.208.0.1 to 204.215.255.254204.215.255.255524,286
28204.216.0.0/13204.216.0.1 to 204.223.255.254204.223.255.255524,286
29204.224.0.0/13204.224.0.1 to 204.231.255.254204.231.255.255524,286
30204.232.0.0/13204.232.0.1 to 204.239.255.254204.239.255.255524,286
31204.240.0.0/13204.240.0.1 to 204.247.255.254204.247.255.255524,286
32204.248.0.0/13204.248.0.1 to 204.255.255.254204.255.255.255524,286
33205.0.0.0/13205.0.0.1 to 205.7.255.254205.7.255.255524,286
34205.8.0.0/13205.8.0.1 to 205.15.255.254205.15.255.255524,286
35205.16.0.0/13205.16.0.1 to 205.23.255.254205.23.255.255524,286
36205.24.0.0/13205.24.0.1 to 205.31.255.254205.31.255.255524,286
37205.32.0.0/13205.32.0.1 to 205.39.255.254205.39.255.255524,286
38205.40.0.0/13205.40.0.1 to 205.47.255.254205.47.255.255524,286
39205.48.0.0/13205.48.0.1 to 205.55.255.254205.55.255.255524,286
40205.56.0.0/13205.56.0.1 to 205.63.255.254205.63.255.255524,286
41205.64.0.0/13205.64.0.1 to 205.71.255.254205.71.255.255524,286
42205.72.0.0/13205.72.0.1 to 205.79.255.254205.79.255.255524,286
43205.80.0.0/13205.80.0.1 to 205.87.255.254205.87.255.255524,286
44205.88.0.0/13205.88.0.1 to 205.95.255.254205.95.255.255524,286
45205.96.0.0/13205.96.0.1 to 205.103.255.254205.103.255.255524,286
46205.104.0.0/13205.104.0.1 to 205.111.255.254205.111.255.255524,286
47205.112.0.0/13205.112.0.1 to 205.119.255.254205.119.255.255524,286
48205.120.0.0/13205.120.0.1 to 205.127.255.254205.127.255.255524,286
49205.128.0.0/13205.128.0.1 to 205.135.255.254205.135.255.255524,286
50205.136.0.0/13205.136.0.1 to 205.143.255.254205.143.255.255524,286
51205.144.0.0/13205.144.0.1 to 205.151.255.254205.151.255.255524,286
52205.152.0.0/13205.152.0.1 to 205.159.255.254205.159.255.255524,286
53205.160.0.0/13205.160.0.1 to 205.167.255.254205.167.255.255524,286
54205.168.0.0/13205.168.0.1 to 205.175.255.254205.175.255.255524,286
55205.176.0.0/13205.176.0.1 to 205.183.255.254205.183.255.255524,286
56205.184.0.0/13205.184.0.1 to 205.191.255.254205.191.255.255524,286
57205.192.0.0/13205.192.0.1 to 205.199.255.254205.199.255.255524,286
58205.200.0.0/13205.200.0.1 to 205.207.255.254205.207.255.255524,286
59205.208.0.0/13205.208.0.1 to 205.215.255.254205.215.255.255524,286
60205.216.0.0/13205.216.0.1 to 205.223.255.254205.223.255.255524,286
61205.224.0.0/13205.224.0.1 to 205.231.255.254205.231.255.255524,286
62205.232.0.0/13205.232.0.1 to 205.239.255.254205.239.255.255524,286
63205.240.0.0/13205.240.0.1 to 205.247.255.254205.247.255.255524,286
64205.248.0.0/13205.248.0.1 to 205.255.255.254205.255.255.255524,286
65206.0.0.0/13206.0.0.1 to 206.7.255.254206.7.255.255524,286
66206.8.0.0/13206.8.0.1 to 206.15.255.254206.15.255.255524,286
67206.16.0.0/13206.16.0.1 to 206.23.255.254206.23.255.255524,286
68206.24.0.0/13206.24.0.1 to 206.31.255.254206.31.255.255524,286
69206.32.0.0/13206.32.0.1 to 206.39.255.254206.39.255.255524,286
70206.40.0.0/13206.40.0.1 to 206.47.255.254206.47.255.255524,286
71206.48.0.0/13206.48.0.1 to 206.55.255.254206.55.255.255524,286
72206.56.0.0/13206.56.0.1 to 206.63.255.254206.63.255.255524,286
73206.64.0.0/13206.64.0.1 to 206.71.255.254206.71.255.255524,286
74206.72.0.0/13206.72.0.1 to 206.79.255.254206.79.255.255524,286
75206.80.0.0/13206.80.0.1 to 206.87.255.254206.87.255.255524,286
76206.88.0.0/13206.88.0.1 to 206.95.255.254206.95.255.255524,286
77206.96.0.0/13206.96.0.1 to 206.103.255.254206.103.255.255524,286
78206.104.0.0/13206.104.0.1 to 206.111.255.254206.111.255.255524,286
79206.112.0.0/13206.112.0.1 to 206.119.255.254206.119.255.255524,286
80206.120.0.0/13206.120.0.1 to 206.127.255.254206.127.255.255524,286
81206.128.0.0/13206.128.0.1 to 206.135.255.254206.135.255.255524,286
82206.136.0.0/13206.136.0.1 to 206.143.255.254206.143.255.255524,286
83206.144.0.0/13206.144.0.1 to 206.151.255.254206.151.255.255524,286
84206.152.0.0/13206.152.0.1 to 206.159.255.254206.159.255.255524,286
85206.160.0.0/13206.160.0.1 to 206.167.255.254206.167.255.255524,286
86206.168.0.0/13206.168.0.1 to 206.175.255.254206.175.255.255524,286
87206.176.0.0/13206.176.0.1 to 206.183.255.254206.183.255.255524,286
88206.184.0.0/13206.184.0.1 to 206.191.255.254206.191.255.255524,286
89206.192.0.0/13206.192.0.1 to 206.199.255.254206.199.255.255524,286
90206.200.0.0/13206.200.0.1 to 206.207.255.254206.207.255.255524,286
91206.208.0.0/13206.208.0.1 to 206.215.255.254206.215.255.255524,286
92206.216.0.0/13206.216.0.1 to 206.223.255.254206.223.255.255524,286
93206.224.0.0/13206.224.0.1 to 206.231.255.254206.231.255.255524,286
94206.232.0.0/13206.232.0.1 to 206.239.255.254206.239.255.255524,286
95206.240.0.0/13206.240.0.1 to 206.247.255.254206.247.255.255524,286
96206.248.0.0/13206.248.0.1 to 206.255.255.254206.255.255.255524,286
97207.0.0.0/13207.0.0.1 to 207.7.255.254207.7.255.255524,286
98207.8.0.0/13207.8.0.1 to 207.15.255.254207.15.255.255524,286
99207.16.0.0/13207.16.0.1 to 207.23.255.254207.23.255.255524,286
100207.24.0.0/13207.24.0.1 to 207.31.255.254207.31.255.255524,286
101207.32.0.0/13207.32.0.1 to 207.39.255.254207.39.255.255524,286
102207.40.0.0/13207.40.0.1 to 207.47.255.254207.47.255.255524,286
103207.48.0.0/13207.48.0.1 to 207.55.255.254207.55.255.255524,286
104207.56.0.0/13207.56.0.1 to 207.63.255.254207.63.255.255524,286
105207.64.0.0/13207.64.0.1 to 207.71.255.254207.71.255.255524,286
106207.72.0.0/13207.72.0.1 to 207.79.255.254207.79.255.255524,286
107207.80.0.0/13207.80.0.1 to 207.87.255.254207.87.255.255524,286
108207.88.0.0/13207.88.0.1 to 207.95.255.254207.95.255.255524,286
109207.96.0.0/13207.96.0.1 to 207.103.255.254207.103.255.255524,286
110207.104.0.0/13207.104.0.1 to 207.111.255.254207.111.255.255524,286
111207.112.0.0/13207.112.0.1 to 207.119.255.254207.119.255.255524,286
112207.120.0.0/13207.120.0.1 to 207.127.255.254207.127.255.255524,286
113207.128.0.0/13207.128.0.1 to 207.135.255.254207.135.255.255524,286
114207.136.0.0/13207.136.0.1 to 207.143.255.254207.143.255.255524,286
115207.144.0.0/13207.144.0.1 to 207.151.255.254207.151.255.255524,286
116207.152.0.0/13207.152.0.1 to 207.159.255.254207.159.255.255524,286
117207.160.0.0/13207.160.0.1 to 207.167.255.254207.167.255.255524,286
118207.168.0.0/13207.168.0.1 to 207.175.255.254207.175.255.255524,286
119207.176.0.0/13207.176.0.1 to 207.183.255.254207.183.255.255524,286
120207.184.0.0/13207.184.0.1 to 207.191.255.254207.191.255.255524,286
121207.192.0.0/13207.192.0.1 to 207.199.255.254207.199.255.255524,286
122207.200.0.0/13207.200.0.1 to 207.207.255.254207.207.255.255524,286
123207.208.0.0/13207.208.0.1 to 207.215.255.254207.215.255.255524,286
124207.216.0.0/13207.216.0.1 to 207.223.255.254207.223.255.255524,286
125207.224.0.0/13207.224.0.1 to 207.231.255.254207.231.255.255524,286
126207.232.0.0/13207.232.0.1 to 207.239.255.254207.239.255.255524,286
127207.240.0.0/13207.240.0.1 to 207.247.255.254207.247.255.255524,286
128207.248.0.0/13207.248.0.1 to 207.255.255.254207.255.255.255524,286

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.