/23 Subnet Mask (255.255.254.0)

The /23 subnet mask is 255.255.254.0, and this free calculator finds the /23 block that holds any IPv4 address you enter. It runs in your browser, so you do not need an account or sign-up. Give it an IP address and a parent network such as 10.20.0.0/16, and it shows the block’s network address, usable host range and position in the parent, plus a table of every /23 subnet in that parent. The prefix is fixed at /23 on this page, so every result is about /23 blocks, which hold 512 addresses each, with 510 usable hosts.

Use it when two /24 networks have to be merged into one, or to check which /23 an address belongs to.

How to use the /23 subnet mask calculator

1. In IP address, type an IPv4 address such as 10.20.5.77. Leading zeros (like 010) and values above 255 are rejected. You can also type an address with a prefix, such as 192.168.10.45/23; only the address part is used.
2. In Parent network (CIDR), enter the larger network to split into /23 blocks, such as 10.20.0.0/16. Its prefix must be /23 or shorter, or the tool shows an error. If host bits are set, the parent is rounded down to its network address and a warning says so.
3. Read the primary results: the /23 block containing your IP and its usable host range. The rows below show the network address, broadcast address, position in the parent (for example, “Subnet 3 of 128”), mask, wildcard, binary mask, total addresses, usable hosts, block size, blocks per bigger network and number of subnets in the parent.
4. Open How it was calculated under the result to see the formula, the values substituted and the answer.
5. Scroll the subnet table to see every /23 subnet of the parent, with its number, network address, first host, last host and broadcast address.
6. To view later rows of a large parent, open More options and set Start the table at subnet number. The table shows 256 rows at a time, so typing 257 shows subnets 257 to 512.

The preset buttons above the form fill in typical cases from private or documentation address space. Reset restores the starting example.

What is the /23 subnet mask?

A /23 prefix keeps the first 23 bits of the address for the network and leaves 9 bits for hosts. Written out, the mask is 11111111.11111111.11111110.00000000, which is 255.255.254.0 in dotted decimal; the wildcard (inverse) mask used in router access lists is 0.0.1.255. Each /23 block holds 2^9 = 512 addresses. The first is the network address and the last is the broadcast address (RFC 950), so 510 are usable hosts.

How do you find the /23 subnet of an IP address?

The mask stops being 255 in the third octet, and the block size there is 256 − 254 = 2. Round that octet of the address down to a multiple of 2, keep the octets before it and set the octets after it to 0. For 10.20.5.77, the third octet is 5; 5 ÷ 2 rounds down to 2, and 2 × 2 = 4. The network is therefore 10.20.4.0/23, the usable hosts run from 10.20.4.1 to 10.20.5.254, and the broadcast address is 10.20.5.255.

How many /23 subnets fit in a larger network?

Each step of one bit doubles the count, so a /16 holds 2^(23 − 16) = 128 subnets of size /23. In 10.20.0.0/16 they start at 10.20.0.0 and go up in steps of 2 in the third octet, to 10.20.254.0/23; 10.20.4.0/23 is number 3. The table on this page lists them with their first host, last host and broadcast address, 256 rows at a time.

How does a /23 compare with the sizes next to it?

One bit shorter, a /22 doubles the block to 1,024 addresses (1,022 usable hosts); one bit longer, a /24 halves it to 256 addresses (254 usable hosts). Two neighbouring /23 blocks can be joined into one /22 only when the first of them starts on a multiple of 4 in the third octet — 10.20.4.0/22 contains 10.20.4.0/23, 10.20.6.0/23.

How is a /23 written in different configurations?

The same mask appears in several forms depending on the device or file: CIDR notation /23 (most routers, cloud consoles and firewalls), dotted decimal 255.255.254.0 (older interface settings, DHCP), hexadecimal 0xFFFFFE00 (some Unix tools such as ifconfig on BSD), and the wildcard 0.0.1.255 in router access lists and OSPF network statements. They all describe the same 512-address block.

Where is a /23 used?

A /23 joins two neighbouring /24s into one network of 510 usable hosts — a common size for a larger office LAN or a busy Wi-Fi network when one /24 is too small.

Worked examples

Calculated by the tool above with these values.

Frequently asked questions

What is the subnet mask for a /23?

255.255.254.0, with wildcard mask 0.0.1.255. In binary the mask is 11111111.11111111.11111110.00000000.

How many usable hosts does a /23 have?

510. A /23 has 512 addresses in total, and the network and broadcast addresses are not assigned to hosts.

Can any two /24s be joined into a /23?

No. A /23 must start on an even third octet: 10.20.4.0 and 10.20.5.0 form 10.20.4.0/23, but 10.20.5.0 and 10.20.6.0 belong to two different /23 blocks.

What is the first and last usable IP address in a /23?

The first usable address is the network address + 1 and the last is the broadcast address − 1. For 10.20.4.0/23 that is 10.20.4.1 to 10.20.5.254.

What does the warning about host bits mean?

The parent network you typed is not a network address (it has host bits set), so the calculator rounds it down to its network address and says so.

Is the calculator free, and is anything I type uploaded?

Yes, it is free, needs no sign-up, and every calculation runs in your browser; nothing you type is uploaded. Results may be used in any work, including commercial projects, without attribution.

Can I print or share a calculation?

Yes. Print / save as PDF gives a one-page record with the inputs, results, steps and date, and Copy link stores your inputs in the page address so the same calculation reopens later.

Related tools

• /29 subnet mask: Use this when you need small blocks of eight addresses, such as point-to-point links.
• /27 subnet: Use this when you want to split a network into blocks of 32 addresses.
• /22 subnet: Use this when you need a larger block of 1,024 addresses.
• /16 subnet: Use this to see how a /16 parent divides into smaller subnets.
• Subnet cheat sheet: Use this for a quick reference of masks, block sizes and host counts.
• Subnet calculator: Use this to work out any prefix length, not just /23.