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.

Network252.2.0.0/21
Usable hosts2,046
Subnet mask255.255.248.0
Addresses2,048
Network address
252.2.0.0
Broadcast address
252.2.7.255
First usable
252.2.0.1
Last usable
252.2.7.254
Usable hosts
2,046 (2,048 less the network and broadcast addresses)
Subnet mask
255.255.248.0
Wildcard mask
0.0.7.255 (Cisco ACLs, OSPF)
Prefix length
/21
Type
Reserved 240.0.0.0/4
Future use; not usable on most systems
Old class
E (reserved)
As numbers
Hex FC020000 · Integer 4227989504 to 4227991551
Reverse DNS zones
0.2.252.in-addr.arpa
1.2.252.in-addr.arpa
2.2.252.in-addr.arpa
3.2.252.in-addr.arpa
4.2.252.in-addr.arpa
5.2.252.in-addr.arpa
6.2.252.in-addr.arpa
7.2.252.in-addr.arpa
/21 is not on a 8-bit boundary, so it takes 8 /24 zones.
Binary
Binary view
Binary (network bits bold, host bits grey)Dotted
Address11111100.00000010.00000000.00000000252.2.0.0
Subnet mask11111111.11111111.11111000.00000000255.255.248.0
Network11111100.00000010.00000000.00000000252.2.0.0
Broadcast11111100.00000010.00000111.11111111252.2.7.255
Split into smaller subnets/28
/22 (2)/23 (4)/24 (8)/25 (16)/26 (32)/27 (64)/28 (128)/29 (256)/30 (512)/31 (1,024)/32 (2,048)

252.2.0.0/21 makes 128 subnets of /28.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
1252.2.0.0/28252.2.0.1 to 252.2.0.14252.2.0.1514
2252.2.0.16/28252.2.0.17 to 252.2.0.30252.2.0.3114
3252.2.0.32/28252.2.0.33 to 252.2.0.46252.2.0.4714
4252.2.0.48/28252.2.0.49 to 252.2.0.62252.2.0.6314
5252.2.0.64/28252.2.0.65 to 252.2.0.78252.2.0.7914
6252.2.0.80/28252.2.0.81 to 252.2.0.94252.2.0.9514
7252.2.0.96/28252.2.0.97 to 252.2.0.110252.2.0.11114
8252.2.0.112/28252.2.0.113 to 252.2.0.126252.2.0.12714
9252.2.0.128/28252.2.0.129 to 252.2.0.142252.2.0.14314
10252.2.0.144/28252.2.0.145 to 252.2.0.158252.2.0.15914
11252.2.0.160/28252.2.0.161 to 252.2.0.174252.2.0.17514
12252.2.0.176/28252.2.0.177 to 252.2.0.190252.2.0.19114
13252.2.0.192/28252.2.0.193 to 252.2.0.206252.2.0.20714
14252.2.0.208/28252.2.0.209 to 252.2.0.222252.2.0.22314
15252.2.0.224/28252.2.0.225 to 252.2.0.238252.2.0.23914
16252.2.0.240/28252.2.0.241 to 252.2.0.254252.2.0.25514
17252.2.1.0/28252.2.1.1 to 252.2.1.14252.2.1.1514
18252.2.1.16/28252.2.1.17 to 252.2.1.30252.2.1.3114
19252.2.1.32/28252.2.1.33 to 252.2.1.46252.2.1.4714
20252.2.1.48/28252.2.1.49 to 252.2.1.62252.2.1.6314
21252.2.1.64/28252.2.1.65 to 252.2.1.78252.2.1.7914
22252.2.1.80/28252.2.1.81 to 252.2.1.94252.2.1.9514
23252.2.1.96/28252.2.1.97 to 252.2.1.110252.2.1.11114
24252.2.1.112/28252.2.1.113 to 252.2.1.126252.2.1.12714
25252.2.1.128/28252.2.1.129 to 252.2.1.142252.2.1.14314
26252.2.1.144/28252.2.1.145 to 252.2.1.158252.2.1.15914
27252.2.1.160/28252.2.1.161 to 252.2.1.174252.2.1.17514
28252.2.1.176/28252.2.1.177 to 252.2.1.190252.2.1.19114
29252.2.1.192/28252.2.1.193 to 252.2.1.206252.2.1.20714
30252.2.1.208/28252.2.1.209 to 252.2.1.222252.2.1.22314
31252.2.1.224/28252.2.1.225 to 252.2.1.238252.2.1.23914
32252.2.1.240/28252.2.1.241 to 252.2.1.254252.2.1.25514
33252.2.2.0/28252.2.2.1 to 252.2.2.14252.2.2.1514
34252.2.2.16/28252.2.2.17 to 252.2.2.30252.2.2.3114
35252.2.2.32/28252.2.2.33 to 252.2.2.46252.2.2.4714
36252.2.2.48/28252.2.2.49 to 252.2.2.62252.2.2.6314
37252.2.2.64/28252.2.2.65 to 252.2.2.78252.2.2.7914
38252.2.2.80/28252.2.2.81 to 252.2.2.94252.2.2.9514
39252.2.2.96/28252.2.2.97 to 252.2.2.110252.2.2.11114
40252.2.2.112/28252.2.2.113 to 252.2.2.126252.2.2.12714
41252.2.2.128/28252.2.2.129 to 252.2.2.142252.2.2.14314
42252.2.2.144/28252.2.2.145 to 252.2.2.158252.2.2.15914
43252.2.2.160/28252.2.2.161 to 252.2.2.174252.2.2.17514
44252.2.2.176/28252.2.2.177 to 252.2.2.190252.2.2.19114
45252.2.2.192/28252.2.2.193 to 252.2.2.206252.2.2.20714
46252.2.2.208/28252.2.2.209 to 252.2.2.222252.2.2.22314
47252.2.2.224/28252.2.2.225 to 252.2.2.238252.2.2.23914
48252.2.2.240/28252.2.2.241 to 252.2.2.254252.2.2.25514
49252.2.3.0/28252.2.3.1 to 252.2.3.14252.2.3.1514
50252.2.3.16/28252.2.3.17 to 252.2.3.30252.2.3.3114
51252.2.3.32/28252.2.3.33 to 252.2.3.46252.2.3.4714
52252.2.3.48/28252.2.3.49 to 252.2.3.62252.2.3.6314
53252.2.3.64/28252.2.3.65 to 252.2.3.78252.2.3.7914
54252.2.3.80/28252.2.3.81 to 252.2.3.94252.2.3.9514
55252.2.3.96/28252.2.3.97 to 252.2.3.110252.2.3.11114
56252.2.3.112/28252.2.3.113 to 252.2.3.126252.2.3.12714
57252.2.3.128/28252.2.3.129 to 252.2.3.142252.2.3.14314
58252.2.3.144/28252.2.3.145 to 252.2.3.158252.2.3.15914
59252.2.3.160/28252.2.3.161 to 252.2.3.174252.2.3.17514
60252.2.3.176/28252.2.3.177 to 252.2.3.190252.2.3.19114
61252.2.3.192/28252.2.3.193 to 252.2.3.206252.2.3.20714
62252.2.3.208/28252.2.3.209 to 252.2.3.222252.2.3.22314
63252.2.3.224/28252.2.3.225 to 252.2.3.238252.2.3.23914
64252.2.3.240/28252.2.3.241 to 252.2.3.254252.2.3.25514
65252.2.4.0/28252.2.4.1 to 252.2.4.14252.2.4.1514
66252.2.4.16/28252.2.4.17 to 252.2.4.30252.2.4.3114
67252.2.4.32/28252.2.4.33 to 252.2.4.46252.2.4.4714
68252.2.4.48/28252.2.4.49 to 252.2.4.62252.2.4.6314
69252.2.4.64/28252.2.4.65 to 252.2.4.78252.2.4.7914
70252.2.4.80/28252.2.4.81 to 252.2.4.94252.2.4.9514
71252.2.4.96/28252.2.4.97 to 252.2.4.110252.2.4.11114
72252.2.4.112/28252.2.4.113 to 252.2.4.126252.2.4.12714
73252.2.4.128/28252.2.4.129 to 252.2.4.142252.2.4.14314
74252.2.4.144/28252.2.4.145 to 252.2.4.158252.2.4.15914
75252.2.4.160/28252.2.4.161 to 252.2.4.174252.2.4.17514
76252.2.4.176/28252.2.4.177 to 252.2.4.190252.2.4.19114
77252.2.4.192/28252.2.4.193 to 252.2.4.206252.2.4.20714
78252.2.4.208/28252.2.4.209 to 252.2.4.222252.2.4.22314
79252.2.4.224/28252.2.4.225 to 252.2.4.238252.2.4.23914
80252.2.4.240/28252.2.4.241 to 252.2.4.254252.2.4.25514
81252.2.5.0/28252.2.5.1 to 252.2.5.14252.2.5.1514
82252.2.5.16/28252.2.5.17 to 252.2.5.30252.2.5.3114
83252.2.5.32/28252.2.5.33 to 252.2.5.46252.2.5.4714
84252.2.5.48/28252.2.5.49 to 252.2.5.62252.2.5.6314
85252.2.5.64/28252.2.5.65 to 252.2.5.78252.2.5.7914
86252.2.5.80/28252.2.5.81 to 252.2.5.94252.2.5.9514
87252.2.5.96/28252.2.5.97 to 252.2.5.110252.2.5.11114
88252.2.5.112/28252.2.5.113 to 252.2.5.126252.2.5.12714
89252.2.5.128/28252.2.5.129 to 252.2.5.142252.2.5.14314
90252.2.5.144/28252.2.5.145 to 252.2.5.158252.2.5.15914
91252.2.5.160/28252.2.5.161 to 252.2.5.174252.2.5.17514
92252.2.5.176/28252.2.5.177 to 252.2.5.190252.2.5.19114
93252.2.5.192/28252.2.5.193 to 252.2.5.206252.2.5.20714
94252.2.5.208/28252.2.5.209 to 252.2.5.222252.2.5.22314
95252.2.5.224/28252.2.5.225 to 252.2.5.238252.2.5.23914
96252.2.5.240/28252.2.5.241 to 252.2.5.254252.2.5.25514
97252.2.6.0/28252.2.6.1 to 252.2.6.14252.2.6.1514
98252.2.6.16/28252.2.6.17 to 252.2.6.30252.2.6.3114
99252.2.6.32/28252.2.6.33 to 252.2.6.46252.2.6.4714
100252.2.6.48/28252.2.6.49 to 252.2.6.62252.2.6.6314
101252.2.6.64/28252.2.6.65 to 252.2.6.78252.2.6.7914
102252.2.6.80/28252.2.6.81 to 252.2.6.94252.2.6.9514
103252.2.6.96/28252.2.6.97 to 252.2.6.110252.2.6.11114
104252.2.6.112/28252.2.6.113 to 252.2.6.126252.2.6.12714
105252.2.6.128/28252.2.6.129 to 252.2.6.142252.2.6.14314
106252.2.6.144/28252.2.6.145 to 252.2.6.158252.2.6.15914
107252.2.6.160/28252.2.6.161 to 252.2.6.174252.2.6.17514
108252.2.6.176/28252.2.6.177 to 252.2.6.190252.2.6.19114
109252.2.6.192/28252.2.6.193 to 252.2.6.206252.2.6.20714
110252.2.6.208/28252.2.6.209 to 252.2.6.222252.2.6.22314
111252.2.6.224/28252.2.6.225 to 252.2.6.238252.2.6.23914
112252.2.6.240/28252.2.6.241 to 252.2.6.254252.2.6.25514
113252.2.7.0/28252.2.7.1 to 252.2.7.14252.2.7.1514
114252.2.7.16/28252.2.7.17 to 252.2.7.30252.2.7.3114
115252.2.7.32/28252.2.7.33 to 252.2.7.46252.2.7.4714
116252.2.7.48/28252.2.7.49 to 252.2.7.62252.2.7.6314
117252.2.7.64/28252.2.7.65 to 252.2.7.78252.2.7.7914
118252.2.7.80/28252.2.7.81 to 252.2.7.94252.2.7.9514
119252.2.7.96/28252.2.7.97 to 252.2.7.110252.2.7.11114
120252.2.7.112/28252.2.7.113 to 252.2.7.126252.2.7.12714
121252.2.7.128/28252.2.7.129 to 252.2.7.142252.2.7.14314
122252.2.7.144/28252.2.7.145 to 252.2.7.158252.2.7.15914
123252.2.7.160/28252.2.7.161 to 252.2.7.174252.2.7.17514
124252.2.7.176/28252.2.7.177 to 252.2.7.190252.2.7.19114
125252.2.7.192/28252.2.7.193 to 252.2.7.206252.2.7.20714
126252.2.7.208/28252.2.7.209 to 252.2.7.222252.2.7.22314
127252.2.7.224/28252.2.7.225 to 252.2.7.238252.2.7.23914
128252.2.7.240/28252.2.7.241 to 252.2.7.254252.2.7.25514

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.