CIDR / Subnet Mask Reference Table

A reference table of subnet masks, wildcard masks, and usable host counts for every prefix length from /0 to /32. Enter how many hosts you need and the smallest CIDR block that fits is highlighted automatically.

CIDR reference table (/0 to /32)

The subnet mask, wildcard mask, total address count, and usable host count for every prefix length.

Prefix Subnet mask Wildcard mask Total addresses Usable hosts
/00.0.0.0255.255.255.2554,294,967,2964,294,967,294
/1128.0.0.0127.255.255.2552,147,483,6482,147,483,646
/2192.0.0.063.255.255.2551,073,741,8241,073,741,822
/3224.0.0.031.255.255.255536,870,912536,870,910
/4240.0.0.015.255.255.255268,435,456268,435,454
/5248.0.0.07.255.255.255134,217,728134,217,726
/6252.0.0.03.255.255.25567,108,86467,108,862
/7254.0.0.01.255.255.25533,554,43233,554,430
/8255.0.0.00.255.255.25516,777,21616,777,214
/9255.128.0.00.127.255.2558,388,6088,388,606
/10255.192.0.00.63.255.2554,194,3044,194,302
/11255.224.0.00.31.255.2552,097,1522,097,150
/12255.240.0.00.15.255.2551,048,5761,048,574
/13255.248.0.00.7.255.255524,288524,286
/14255.252.0.00.3.255.255262,144262,142
/15255.254.0.00.1.255.255131,072131,070
/16255.255.0.00.0.255.25565,53665,534
/17255.255.128.00.0.127.25532,76832,766
/18255.255.192.00.0.63.25516,38416,382
/19255.255.224.00.0.31.2558,1928,190
/20255.255.240.00.0.15.2554,0964,094
/21255.255.248.00.0.7.2552,0482,046
/22255.255.252.00.0.3.2551,0241,022
/23255.255.254.00.0.1.255512510
/24255.255.255.00.0.0.255256254
/25255.255.255.1280.0.0.127128126
/26255.255.255.1920.0.0.636462
/27255.255.255.2240.0.0.313230
/28255.255.255.2400.0.0.151614
/29255.255.255.2480.0.0.786
/30255.255.255.2520.0.0.342
/31255.255.255.2540.0.0.122
/32255.255.255.2550.0.0.011

Usable hosts excludes the network and broadcast addresses (except /31, which RFC 3021 allows to use both of its 2 addresses, and /32, counted as a single address).

Tips

  • "/24" means 255.255.255.0 (256 addresses, 254 usable hosts) — the most common size for a small office LAN or home network.
  • /31 is a special case where both addresses are usable (RFC 3021). It's commonly used on point-to-point router links to save address space.
  • If you don't know the exact number of hosts you need, enter your headcount or device count in the box above and the smallest CIDR block that fits will be highlighted in the table below.
  • A wildcard mask — the inverse of a subnet mask — is used in Cisco ACL (access control list) rules and OSPF network statements.

FAQ

A /24 contains 256 IP addresses (0–255), but the first address (network) and the last (broadcast) usually can't be assigned to a device, leaving 254 addresses actually usable by hosts.

A subnet mask marks the network portion with 1s and the host portion with 0s (e.g. 255.255.255.0); a wildcard mask is its inverse (e.g. 0.0.0.255). Cisco router access lists and OSPF configuration use wildcard masks, so it helps to know how the two correspond.

RFC 3021 formally allows a /31 to use both of its addresses on point-to-point links such as router-to-router connections. A /32 refers to a single IP address as a host route, used in routing tables to direct traffic to one specific host.

Enter the number of hosts you need in the box at the top of this tool, and it automatically determines the smallest CIDR block (prefix length) that fits, highlighting the matching row in the table below. If you expect future growth, it's common practice to pick a slightly larger block than you strictly need today.
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Side Note — IPv4 address exhaustion and the birth of CIDR

CIDR (Classless Inter-Domain Routing) was introduced in 1993. Before that, networks could only be sized as Class A (/8), Class B (/16), or Class C (/24) — three fixed sizes. Assigning a single Class B reserved 65,534 host addresses, so handing one to an organization that only needed a few thousand wasted vast numbers of addresses; in the early 1990s, this waste was accelerating IPv4 address exhaustion significantly.

CIDR solved this by letting the prefix length be chosen freely, so a block could be sized much closer to the actual number of hosts needed. This dramatically improved address-space efficiency and extended IPv4's practical lifespan well beyond what had been expected. CIDR also made it possible to summarize multiple contiguous networks into a single route — "route aggregation" — which helped keep the internet's global routing tables from growing out of control.

New public IPv4 allocations have largely run out today, and the industry keeps moving toward IPv6, but CIDR notation is still used daily to design subnets inside corporate LANs and cloud VPCs. Deciding "do I need a block a bit bigger than a /24?" remains a routine, practical skill for cloud engineers.

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