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.

Network37.2.136.0/21

Usable hosts2,046

Subnet mask255.255.248.0

Addresses2,048

Network address: 37.2.136.0
Broadcast address: 37.2.143.255
First usable: 37.2.136.1
Last usable: 37.2.143.254
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 25028800 · Integer 620922880 to 620924927
Reverse DNS zones: 136.2.37.in-addr.arpa
137.2.37.in-addr.arpa
138.2.37.in-addr.arpa
139.2.37.in-addr.arpa
140.2.37.in-addr.arpa
141.2.37.in-addr.arpa
142.2.37.in-addr.arpa
143.2.37.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
Address	`00100101.00000010.10001000.00000000`	`37.2.136.0`
Subnet mask	`11111111.11111111.11111000.00000000`	`255.255.248.0`
Network	`00100101.00000010.10001000.00000000`	`37.2.136.0`
Broadcast	`00100101.00000010.10001111.11111111`	`37.2.143.255`

Split into smaller subnets/27

/22 (2)/23 (4)/24 (8)/25 (16)/26 (32)/27 (64)/28 (128)/29 (256)/30 (512)/31 (1,024)/32 (2,048)

37.2.136.0/21 makes 64 subnets of /27.

Subnets
#	Subnet	Usable hosts range	Broadcast	Hosts
1	`37.2.136.0/27`	`37.2.136.1` to `37.2.136.30`	37.2.136.31	30
2	`37.2.136.32/27`	`37.2.136.33` to `37.2.136.62`	37.2.136.63	30
3	`37.2.136.64/27`	`37.2.136.65` to `37.2.136.94`	37.2.136.95	30
4	`37.2.136.96/27`	`37.2.136.97` to `37.2.136.126`	37.2.136.127	30
5	`37.2.136.128/27`	`37.2.136.129` to `37.2.136.158`	37.2.136.159	30
6	`37.2.136.160/27`	`37.2.136.161` to `37.2.136.190`	37.2.136.191	30
7	`37.2.136.192/27`	`37.2.136.193` to `37.2.136.222`	37.2.136.223	30
8	`37.2.136.224/27`	`37.2.136.225` to `37.2.136.254`	37.2.136.255	30
9	`37.2.137.0/27`	`37.2.137.1` to `37.2.137.30`	37.2.137.31	30
10	`37.2.137.32/27`	`37.2.137.33` to `37.2.137.62`	37.2.137.63	30
11	`37.2.137.64/27`	`37.2.137.65` to `37.2.137.94`	37.2.137.95	30
12	`37.2.137.96/27`	`37.2.137.97` to `37.2.137.126`	37.2.137.127	30
13	`37.2.137.128/27`	`37.2.137.129` to `37.2.137.158`	37.2.137.159	30
14	`37.2.137.160/27`	`37.2.137.161` to `37.2.137.190`	37.2.137.191	30
15	`37.2.137.192/27`	`37.2.137.193` to `37.2.137.222`	37.2.137.223	30
16	`37.2.137.224/27`	`37.2.137.225` to `37.2.137.254`	37.2.137.255	30
17	`37.2.138.0/27`	`37.2.138.1` to `37.2.138.30`	37.2.138.31	30
18	`37.2.138.32/27`	`37.2.138.33` to `37.2.138.62`	37.2.138.63	30
19	`37.2.138.64/27`	`37.2.138.65` to `37.2.138.94`	37.2.138.95	30
20	`37.2.138.96/27`	`37.2.138.97` to `37.2.138.126`	37.2.138.127	30
21	`37.2.138.128/27`	`37.2.138.129` to `37.2.138.158`	37.2.138.159	30
22	`37.2.138.160/27`	`37.2.138.161` to `37.2.138.190`	37.2.138.191	30
23	`37.2.138.192/27`	`37.2.138.193` to `37.2.138.222`	37.2.138.223	30
24	`37.2.138.224/27`	`37.2.138.225` to `37.2.138.254`	37.2.138.255	30
25	`37.2.139.0/27`	`37.2.139.1` to `37.2.139.30`	37.2.139.31	30
26	`37.2.139.32/27`	`37.2.139.33` to `37.2.139.62`	37.2.139.63	30
27	`37.2.139.64/27`	`37.2.139.65` to `37.2.139.94`	37.2.139.95	30
28	`37.2.139.96/27`	`37.2.139.97` to `37.2.139.126`	37.2.139.127	30
29	`37.2.139.128/27`	`37.2.139.129` to `37.2.139.158`	37.2.139.159	30
30	`37.2.139.160/27`	`37.2.139.161` to `37.2.139.190`	37.2.139.191	30
31	`37.2.139.192/27`	`37.2.139.193` to `37.2.139.222`	37.2.139.223	30
32	`37.2.139.224/27`	`37.2.139.225` to `37.2.139.254`	37.2.139.255	30
33	`37.2.140.0/27`	`37.2.140.1` to `37.2.140.30`	37.2.140.31	30
34	`37.2.140.32/27`	`37.2.140.33` to `37.2.140.62`	37.2.140.63	30
35	`37.2.140.64/27`	`37.2.140.65` to `37.2.140.94`	37.2.140.95	30
36	`37.2.140.96/27`	`37.2.140.97` to `37.2.140.126`	37.2.140.127	30
37	`37.2.140.128/27`	`37.2.140.129` to `37.2.140.158`	37.2.140.159	30
38	`37.2.140.160/27`	`37.2.140.161` to `37.2.140.190`	37.2.140.191	30
39	`37.2.140.192/27`	`37.2.140.193` to `37.2.140.222`	37.2.140.223	30
40	`37.2.140.224/27`	`37.2.140.225` to `37.2.140.254`	37.2.140.255	30
41	`37.2.141.0/27`	`37.2.141.1` to `37.2.141.30`	37.2.141.31	30
42	`37.2.141.32/27`	`37.2.141.33` to `37.2.141.62`	37.2.141.63	30
43	`37.2.141.64/27`	`37.2.141.65` to `37.2.141.94`	37.2.141.95	30
44	`37.2.141.96/27`	`37.2.141.97` to `37.2.141.126`	37.2.141.127	30
45	`37.2.141.128/27`	`37.2.141.129` to `37.2.141.158`	37.2.141.159	30
46	`37.2.141.160/27`	`37.2.141.161` to `37.2.141.190`	37.2.141.191	30
47	`37.2.141.192/27`	`37.2.141.193` to `37.2.141.222`	37.2.141.223	30
48	`37.2.141.224/27`	`37.2.141.225` to `37.2.141.254`	37.2.141.255	30
49	`37.2.142.0/27`	`37.2.142.1` to `37.2.142.30`	37.2.142.31	30
50	`37.2.142.32/27`	`37.2.142.33` to `37.2.142.62`	37.2.142.63	30
51	`37.2.142.64/27`	`37.2.142.65` to `37.2.142.94`	37.2.142.95	30
52	`37.2.142.96/27`	`37.2.142.97` to `37.2.142.126`	37.2.142.127	30
53	`37.2.142.128/27`	`37.2.142.129` to `37.2.142.158`	37.2.142.159	30
54	`37.2.142.160/27`	`37.2.142.161` to `37.2.142.190`	37.2.142.191	30
55	`37.2.142.192/27`	`37.2.142.193` to `37.2.142.222`	37.2.142.223	30
56	`37.2.142.224/27`	`37.2.142.225` to `37.2.142.254`	37.2.142.255	30
57	`37.2.143.0/27`	`37.2.143.1` to `37.2.143.30`	37.2.143.31	30
58	`37.2.143.32/27`	`37.2.143.33` to `37.2.143.62`	37.2.143.63	30
59	`37.2.143.64/27`	`37.2.143.65` to `37.2.143.94`	37.2.143.95	30
60	`37.2.143.96/27`	`37.2.143.97` to `37.2.143.126`	37.2.143.127	30
61	`37.2.143.128/27`	`37.2.143.129` to `37.2.143.158`	37.2.143.159	30
62	`37.2.143.160/27`	`37.2.143.161` to `37.2.143.190`	37.2.143.191	30
63	`37.2.143.192/27`	`37.2.143.193` to `37.2.143.222`	37.2.143.223	30
64	`37.2.143.224/27`	`37.2.143.225` to `37.2.143.254`	37.2.143.255	30

IPv4 CIDR reference (/0 to /32)

IPv4 prefix lengths
Prefix	Subnet mask	Wildcard	Addresses	Usable hosts	Size	Typical use
/0	0.0.0.0	255.255.255.255	4,294,967,296	4,294,967,294	16,777,216 × /24
/1	128.0.0.0	127.255.255.255	2,147,483,648	2,147,483,646	8,388,608 × /24
/2	192.0.0.0	63.255.255.255	1,073,741,824	1,073,741,822	4,194,304 × /24
/3	224.0.0.0	31.255.255.255	536,870,912	536,870,910	2,097,152 × /24
/4	240.0.0.0	15.255.255.255	268,435,456	268,435,454	1,048,576 × /24
/5	248.0.0.0	7.255.255.255	134,217,728	134,217,726	524,288 × /24
/6	252.0.0.0	3.255.255.255	67,108,864	67,108,862	262,144 × /24
/7	254.0.0.0	1.255.255.255	33,554,432	33,554,430	131,072 × /24
/8	255.0.0.0	0.255.255.255	16,777,216	16,777,214	65,536 × /24	Old class A; 10.0.0.0/8
/9	255.128.0.0	0.127.255.255	8,388,608	8,388,606	32,768 × /24
/10	255.192.0.0	0.63.255.255	4,194,304	4,194,302	16,384 × /24
/11	255.224.0.0	0.31.255.255	2,097,152	2,097,150	8,192 × /24
/12	255.240.0.0	0.15.255.255	1,048,576	1,048,574	4,096 × /24	172.16.0.0/12
/13	255.248.0.0	0.7.255.255	524,288	524,286	2,048 × /24
/14	255.252.0.0	0.3.255.255	262,144	262,142	1,024 × /24
/15	255.254.0.0	0.1.255.255	131,072	131,070	512 × /24
/16	255.255.0.0	0.0.255.255	65,536	65,534	256 × /24	Old class B; 192.168.0.0/16
/17	255.255.128.0	0.0.127.255	32,768	32,766	128 × /24
/18	255.255.192.0	0.0.63.255	16,384	16,382	64 × /24
/19	255.255.224.0	0.0.31.255	8,192	8,190	32 × /24
/20	255.255.240.0	0.0.15.255	4,096	4,094	16 × /24
/21	255.255.248.0	0.0.7.255	2,048	2,046	8 × /24
/22	255.255.252.0	0.0.3.255	1,024	1,022	4 × /24	Large office or guest Wi-Fi (1,022 hosts)
/23	255.255.254.0	0.0.1.255	512	510	2 × /24	Larger office LAN (510 hosts)
/24	255.255.255.0	0.0.0.255	256	254	1 × /24	Typical office LAN (old class C)
/25	255.255.255.128	0.0.0.127	128	126	1/2 of a /24	Smaller LAN or VLAN
/26	255.255.255.192	0.0.0.63	64	62	1/4 of a /24	Smaller LAN or VLAN
/27	255.255.255.224	0.0.0.31	32	30	1/8 of a /24	Small VLAN, phones, cameras
/28	255.255.255.240	0.0.0.15	16	14	1/16 of a /24	Small VLAN, phones, cameras
/29	255.255.255.248	0.0.0.7	8	6	1/32 of a /24	Small fixed public block (5 usable)
/30	255.255.255.252	0.0.0.3	4	2	1/64 of a /24	Point-to-point (old style)
/31	255.255.255.254	0.0.0.1	2	2	1/128 of a /24	Point-to-point link (RFC 3021)
/32	255.255.255.255	0.0.0.0	1	1	1/256 of a /24	One host

IPv6 prefix reference

IPv6 prefix lengths
Prefix	Addresses	/64 LANs	/48 sites	Nibble	Typical use
/3	42,535,295,865,117,307,932,921,825,928,971,026,432 (2^125)	2,305,843,009,213,693,952	35,184,372,088,832	No	All global unicast (2000::/3)
/16	5,192,296,858,534,827,628,530,496,329,220,096 (2^112)	281,474,976,710,656	4,294,967,296	Yes
/20	324,518,553,658,426,726,783,156,020,576,256 (2^108)	17,592,186,044,416	268,435,456	Yes	Large provider allocation (RIPE minimum for some)
/29	633,825,300,114,114,700,748,351,602,688 (2^99)	34,359,738,368	524,288	No	RIPE allocation to a provider (often)
/32	79,228,162,514,264,337,593,543,950,336 (2^96)	4,294,967,296	65,536	Yes	Provider allocation (RIPE default)
/36	4,951,760,157,141,521,099,596,496,896 (2^92)	268,435,456	4,096	Yes
/40	309,485,009,821,345,068,724,781,056 (2^88)	16,777,216	256	Yes
/44	19,342,813,113,834,066,795,298,816 (2^84)	1,048,576	16	Yes
/48	1,208,925,819,614,629,174,706,176 (2^80)	65,536	1	Yes	One site or organisation (RIPE PI / assignment)
/52	75,557,863,725,914,323,419,136 (2^76)	4,096		Yes
/56	4,722,366,482,869,645,213,696 (2^72)	256		Yes	Small site or home (common provider assignment)
/60	295,147,905,179,352,825,856 (2^68)	16		Yes	Some home broadband (16 LANs)
/64	18,446,744,073,709,551,616 (2^64)	1		Yes	One LAN or VLAN (needed for SLAAC)
/72	72,057,594,037,927,936 (2^56)	1/256 of a /64		Yes
/80	281,474,976,710,656 (2^48)	1/65,536 of a /64		Yes
/96	4,294,967,296	1/4,294,967,296 of a /64		Yes	Embeds an IPv4 address (NAT64)
/112	65,536	1/281,474,976,710,656 of a /64		Yes
/120	256	1/72,057,594,037,927,936 of a /64		Yes
/124	16	1/1,152,921,504,606,846,976 of a /64		Yes
/126	4	1/4,611,686,018,427,387,904 of a /64		No
/127	2	1/9,223,372,036,854,775,808 of a /64		No	Point-to-point link (RFC 6164)
/128	1	1/18,446,744,073,709,551,616 of a /64		Yes	One host or loopback

Private and special ranges

IPv4

Special-purpose ranges
Range	Name	What it is
`255.255.255.255/32`	Limited broadcast	Every host on the local network
`0.0.0.0/8`	This network	Not routed; 0.0.0.0 means "any address" in settings
`10.0.0.0/8`	Private (RFC 1918)	Office and home networks; not routed on the internet
`100.64.0.0/10`	Shared address space (CGNAT)	Used by providers behind carrier-grade NAT (RFC 6598)
`127.0.0.0/8`	Loopback	This computer
`169.254.0.0/16`	Link-local (APIPA)	Self-assigned when no DHCP server answers
`172.16.0.0/12`	Private (RFC 1918)	Office and home networks; not routed on the internet
`192.0.0.0/24`	IETF protocol assignments	Special uses such as DS-Lite
`192.0.2.0/24`	Documentation (TEST-NET-1)	For examples; never used on a network
`192.88.99.0/24`	6to4 relay (deprecated)	Formerly anycast for 6to4
`192.168.0.0/16`	Private (RFC 1918)	Office and home networks; not routed on the internet
`198.18.0.0/15`	Benchmarking	Network equipment testing (RFC 2544)
`198.51.100.0/24`	Documentation (TEST-NET-2)	For examples; never used on a network
`203.0.113.0/24`	Documentation (TEST-NET-3)	For examples; never used on a network
`224.0.0.0/4`	Multicast	One-to-many traffic, such as IPTV and routing protocols
`240.0.0.0/4`	Reserved	Future use; not usable on most systems

IPv6

Special-purpose ranges
Range	Name	What it is
`::/128`	Unspecified	No address; :: means "any address" in settings
`::1/128`	Loopback	This computer
`::ffff:0:0/96`	IPv4-mapped	An IPv4 address written as IPv6
`64:ff9b::/96`	NAT64	IPv4 addresses reached through a NAT64 gateway
`100::/64`	Discard only	Traffic sent here is dropped (RFC 6666)
`2001:db8::/32`	Documentation	For examples; never used on a network
`2001::/32`	Teredo	IPv6 tunnelled over IPv4 UDP
`2002::/16`	6to4	IPv6 tunnelled over IPv4 (largely retired)
`3fff::/20`	Documentation	For examples (RFC 9637)
`fc00::/7`	Unique local (ULA)	Private IPv6, like RFC 1918; usually fd00::/8
`fe80::/10`	Link-local	Every IPv6 interface has one; not routed
`ff00::/8`	Multicast	One-to-many traffic; replaces broadcast
`2000::/3`	Global unicast	Public 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.

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IP subnet calculator

IPv4 CIDR reference (/0 to /32)

IPv6 prefix reference

Private and special ranges

IPv4

IPv6

About this tool

Common questions

IPv4 CIDR reference (/0 to /32)

IPv6 prefix reference

Private and special ranges

IPv4

IPv6

Common questions

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