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.

Network30.2.40.0/21
Usable hosts2,046
Subnet mask255.255.248.0
Addresses2,048
Network address
30.2.40.0
Broadcast address
30.2.47.255
First usable
30.2.40.1
Last usable
30.2.47.254
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
Public public
Old class
A (default /8)
As numbers
Hex 1E022800 · Integer 503457792 to 503459839
Reverse DNS zones
40.2.30.in-addr.arpa
41.2.30.in-addr.arpa
42.2.30.in-addr.arpa
43.2.30.in-addr.arpa
44.2.30.in-addr.arpa
45.2.30.in-addr.arpa
46.2.30.in-addr.arpa
47.2.30.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
Address00011110.00000010.00101000.0000000030.2.40.0
Subnet mask11111111.11111111.11111000.00000000255.255.248.0
Network00011110.00000010.00101000.0000000030.2.40.0
Broadcast00011110.00000010.00101111.1111111130.2.47.255
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)

30.2.40.0/21 makes 128 subnets of /28.

Subnets
#SubnetUsable hosts rangeBroadcastHosts
130.2.40.0/2830.2.40.1 to 30.2.40.1430.2.40.1514
230.2.40.16/2830.2.40.17 to 30.2.40.3030.2.40.3114
330.2.40.32/2830.2.40.33 to 30.2.40.4630.2.40.4714
430.2.40.48/2830.2.40.49 to 30.2.40.6230.2.40.6314
530.2.40.64/2830.2.40.65 to 30.2.40.7830.2.40.7914
630.2.40.80/2830.2.40.81 to 30.2.40.9430.2.40.9514
730.2.40.96/2830.2.40.97 to 30.2.40.11030.2.40.11114
830.2.40.112/2830.2.40.113 to 30.2.40.12630.2.40.12714
930.2.40.128/2830.2.40.129 to 30.2.40.14230.2.40.14314
1030.2.40.144/2830.2.40.145 to 30.2.40.15830.2.40.15914
1130.2.40.160/2830.2.40.161 to 30.2.40.17430.2.40.17514
1230.2.40.176/2830.2.40.177 to 30.2.40.19030.2.40.19114
1330.2.40.192/2830.2.40.193 to 30.2.40.20630.2.40.20714
1430.2.40.208/2830.2.40.209 to 30.2.40.22230.2.40.22314
1530.2.40.224/2830.2.40.225 to 30.2.40.23830.2.40.23914
1630.2.40.240/2830.2.40.241 to 30.2.40.25430.2.40.25514
1730.2.41.0/2830.2.41.1 to 30.2.41.1430.2.41.1514
1830.2.41.16/2830.2.41.17 to 30.2.41.3030.2.41.3114
1930.2.41.32/2830.2.41.33 to 30.2.41.4630.2.41.4714
2030.2.41.48/2830.2.41.49 to 30.2.41.6230.2.41.6314
2130.2.41.64/2830.2.41.65 to 30.2.41.7830.2.41.7914
2230.2.41.80/2830.2.41.81 to 30.2.41.9430.2.41.9514
2330.2.41.96/2830.2.41.97 to 30.2.41.11030.2.41.11114
2430.2.41.112/2830.2.41.113 to 30.2.41.12630.2.41.12714
2530.2.41.128/2830.2.41.129 to 30.2.41.14230.2.41.14314
2630.2.41.144/2830.2.41.145 to 30.2.41.15830.2.41.15914
2730.2.41.160/2830.2.41.161 to 30.2.41.17430.2.41.17514
2830.2.41.176/2830.2.41.177 to 30.2.41.19030.2.41.19114
2930.2.41.192/2830.2.41.193 to 30.2.41.20630.2.41.20714
3030.2.41.208/2830.2.41.209 to 30.2.41.22230.2.41.22314
3130.2.41.224/2830.2.41.225 to 30.2.41.23830.2.41.23914
3230.2.41.240/2830.2.41.241 to 30.2.41.25430.2.41.25514
3330.2.42.0/2830.2.42.1 to 30.2.42.1430.2.42.1514
3430.2.42.16/2830.2.42.17 to 30.2.42.3030.2.42.3114
3530.2.42.32/2830.2.42.33 to 30.2.42.4630.2.42.4714
3630.2.42.48/2830.2.42.49 to 30.2.42.6230.2.42.6314
3730.2.42.64/2830.2.42.65 to 30.2.42.7830.2.42.7914
3830.2.42.80/2830.2.42.81 to 30.2.42.9430.2.42.9514
3930.2.42.96/2830.2.42.97 to 30.2.42.11030.2.42.11114
4030.2.42.112/2830.2.42.113 to 30.2.42.12630.2.42.12714
4130.2.42.128/2830.2.42.129 to 30.2.42.14230.2.42.14314
4230.2.42.144/2830.2.42.145 to 30.2.42.15830.2.42.15914
4330.2.42.160/2830.2.42.161 to 30.2.42.17430.2.42.17514
4430.2.42.176/2830.2.42.177 to 30.2.42.19030.2.42.19114
4530.2.42.192/2830.2.42.193 to 30.2.42.20630.2.42.20714
4630.2.42.208/2830.2.42.209 to 30.2.42.22230.2.42.22314
4730.2.42.224/2830.2.42.225 to 30.2.42.23830.2.42.23914
4830.2.42.240/2830.2.42.241 to 30.2.42.25430.2.42.25514
4930.2.43.0/2830.2.43.1 to 30.2.43.1430.2.43.1514
5030.2.43.16/2830.2.43.17 to 30.2.43.3030.2.43.3114
5130.2.43.32/2830.2.43.33 to 30.2.43.4630.2.43.4714
5230.2.43.48/2830.2.43.49 to 30.2.43.6230.2.43.6314
5330.2.43.64/2830.2.43.65 to 30.2.43.7830.2.43.7914
5430.2.43.80/2830.2.43.81 to 30.2.43.9430.2.43.9514
5530.2.43.96/2830.2.43.97 to 30.2.43.11030.2.43.11114
5630.2.43.112/2830.2.43.113 to 30.2.43.12630.2.43.12714
5730.2.43.128/2830.2.43.129 to 30.2.43.14230.2.43.14314
5830.2.43.144/2830.2.43.145 to 30.2.43.15830.2.43.15914
5930.2.43.160/2830.2.43.161 to 30.2.43.17430.2.43.17514
6030.2.43.176/2830.2.43.177 to 30.2.43.19030.2.43.19114
6130.2.43.192/2830.2.43.193 to 30.2.43.20630.2.43.20714
6230.2.43.208/2830.2.43.209 to 30.2.43.22230.2.43.22314
6330.2.43.224/2830.2.43.225 to 30.2.43.23830.2.43.23914
6430.2.43.240/2830.2.43.241 to 30.2.43.25430.2.43.25514
6530.2.44.0/2830.2.44.1 to 30.2.44.1430.2.44.1514
6630.2.44.16/2830.2.44.17 to 30.2.44.3030.2.44.3114
6730.2.44.32/2830.2.44.33 to 30.2.44.4630.2.44.4714
6830.2.44.48/2830.2.44.49 to 30.2.44.6230.2.44.6314
6930.2.44.64/2830.2.44.65 to 30.2.44.7830.2.44.7914
7030.2.44.80/2830.2.44.81 to 30.2.44.9430.2.44.9514
7130.2.44.96/2830.2.44.97 to 30.2.44.11030.2.44.11114
7230.2.44.112/2830.2.44.113 to 30.2.44.12630.2.44.12714
7330.2.44.128/2830.2.44.129 to 30.2.44.14230.2.44.14314
7430.2.44.144/2830.2.44.145 to 30.2.44.15830.2.44.15914
7530.2.44.160/2830.2.44.161 to 30.2.44.17430.2.44.17514
7630.2.44.176/2830.2.44.177 to 30.2.44.19030.2.44.19114
7730.2.44.192/2830.2.44.193 to 30.2.44.20630.2.44.20714
7830.2.44.208/2830.2.44.209 to 30.2.44.22230.2.44.22314
7930.2.44.224/2830.2.44.225 to 30.2.44.23830.2.44.23914
8030.2.44.240/2830.2.44.241 to 30.2.44.25430.2.44.25514
8130.2.45.0/2830.2.45.1 to 30.2.45.1430.2.45.1514
8230.2.45.16/2830.2.45.17 to 30.2.45.3030.2.45.3114
8330.2.45.32/2830.2.45.33 to 30.2.45.4630.2.45.4714
8430.2.45.48/2830.2.45.49 to 30.2.45.6230.2.45.6314
8530.2.45.64/2830.2.45.65 to 30.2.45.7830.2.45.7914
8630.2.45.80/2830.2.45.81 to 30.2.45.9430.2.45.9514
8730.2.45.96/2830.2.45.97 to 30.2.45.11030.2.45.11114
8830.2.45.112/2830.2.45.113 to 30.2.45.12630.2.45.12714
8930.2.45.128/2830.2.45.129 to 30.2.45.14230.2.45.14314
9030.2.45.144/2830.2.45.145 to 30.2.45.15830.2.45.15914
9130.2.45.160/2830.2.45.161 to 30.2.45.17430.2.45.17514
9230.2.45.176/2830.2.45.177 to 30.2.45.19030.2.45.19114
9330.2.45.192/2830.2.45.193 to 30.2.45.20630.2.45.20714
9430.2.45.208/2830.2.45.209 to 30.2.45.22230.2.45.22314
9530.2.45.224/2830.2.45.225 to 30.2.45.23830.2.45.23914
9630.2.45.240/2830.2.45.241 to 30.2.45.25430.2.45.25514
9730.2.46.0/2830.2.46.1 to 30.2.46.1430.2.46.1514
9830.2.46.16/2830.2.46.17 to 30.2.46.3030.2.46.3114
9930.2.46.32/2830.2.46.33 to 30.2.46.4630.2.46.4714
10030.2.46.48/2830.2.46.49 to 30.2.46.6230.2.46.6314
10130.2.46.64/2830.2.46.65 to 30.2.46.7830.2.46.7914
10230.2.46.80/2830.2.46.81 to 30.2.46.9430.2.46.9514
10330.2.46.96/2830.2.46.97 to 30.2.46.11030.2.46.11114
10430.2.46.112/2830.2.46.113 to 30.2.46.12630.2.46.12714
10530.2.46.128/2830.2.46.129 to 30.2.46.14230.2.46.14314
10630.2.46.144/2830.2.46.145 to 30.2.46.15830.2.46.15914
10730.2.46.160/2830.2.46.161 to 30.2.46.17430.2.46.17514
10830.2.46.176/2830.2.46.177 to 30.2.46.19030.2.46.19114
10930.2.46.192/2830.2.46.193 to 30.2.46.20630.2.46.20714
11030.2.46.208/2830.2.46.209 to 30.2.46.22230.2.46.22314
11130.2.46.224/2830.2.46.225 to 30.2.46.23830.2.46.23914
11230.2.46.240/2830.2.46.241 to 30.2.46.25430.2.46.25514
11330.2.47.0/2830.2.47.1 to 30.2.47.1430.2.47.1514
11430.2.47.16/2830.2.47.17 to 30.2.47.3030.2.47.3114
11530.2.47.32/2830.2.47.33 to 30.2.47.4630.2.47.4714
11630.2.47.48/2830.2.47.49 to 30.2.47.6230.2.47.6314
11730.2.47.64/2830.2.47.65 to 30.2.47.7830.2.47.7914
11830.2.47.80/2830.2.47.81 to 30.2.47.9430.2.47.9514
11930.2.47.96/2830.2.47.97 to 30.2.47.11030.2.47.11114
12030.2.47.112/2830.2.47.113 to 30.2.47.12630.2.47.12714
12130.2.47.128/2830.2.47.129 to 30.2.47.14230.2.47.14314
12230.2.47.144/2830.2.47.145 to 30.2.47.15830.2.47.15914
12330.2.47.160/2830.2.47.161 to 30.2.47.17430.2.47.17514
12430.2.47.176/2830.2.47.177 to 30.2.47.19030.2.47.19114
12530.2.47.192/2830.2.47.193 to 30.2.47.20630.2.47.20714
12630.2.47.208/2830.2.47.209 to 30.2.47.22230.2.47.22314
12730.2.47.224/2830.2.47.225 to 30.2.47.23830.2.47.23914
12830.2.47.240/2830.2.47.241 to 30.2.47.25430.2.47.25514

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.