CIDR Calculator & Subnet VLSM Visualizer

About CIDR Calculator & Subnet VLSM Visualizer

This tool takes an IPv4 address with a prefix length (or a netmask) and works out the network address, broadcast, usable host range and wildcard mask, and can split a network into equal smaller subnets in a table. The Subnet Visualizer tab places several subnets of different sizes inside one parent network using VLSM and draws a bar showing which parts of the address space are used and which are free.

Every result comes from treating the address as a single 32-bit integer and doing bitwise AND and OR. Below, the arithmetic is worked through once by hand, followed by the /31 and /32 exceptions, the private ranges, and why IPv6 networks rarely need this kind of calculation.

Working a /26 by hand

Take 192.168.10.77/26. A /26 means the first 26 bits are the network and the last 6 bits are the host, so each block is 2^6 = 64 addresses. The netmask has 26 one-bits, 255.255.255.192, and blocks in the last octet start at 0, 64, 128 and 192.

77 lies between 64 and 128, so the network address is 192.168.10.64 and the broadcast, the last address before the next block, is 192.168.10.127. Remove those two and .65 through .126 remain: 62 usable host addresses. In general, usable hosts = 2^(32 − prefix) − 2. The wildcard mask (0.0.0.63) is the netmask inverted; Cisco ACLs and OSPF network statements ask for it in that form.

A quick mental shortcut: 256 minus the last non-zero netmask octet gives the block size. 255.255.255.224 means blocks of 32; 255.255.255.240 means blocks of 16.

/31 and /32 play by different rules

The "minus two" is there because the network and broadcast addresses cannot be assigned to hosts. Applied blindly, a /31 would have 2 − 2 = 0 usable addresses. RFC 3021 changed that for point-to-point links: with only two routers on a link, a broadcast address is pointless, so both addresses of a /31 may be used. Compared with the older habit of giving every router link a /30 (four addresses, two usable), it halves the address cost.

A /32 is a single address, used for loopback interfaces and host routes. The tool reports two usable addresses for a /31 (both the first and last) and one for a /32. Do not build an ordinary LAN on a /31, though; not every device and operating system handles a subnet without a broadcast address.

Private ranges (RFC 1918) and a common slip

RFC 1918 reserves three IPv4 blocks for private networks. They are not routed on the public internet and anyone may use them internally.

• 10.0.0.0/8 — 10.0.0.0 to 10.255.255.255 (about 16.8 million addresses)
• 172.16.0.0/12 — 172.16.0.0 to 172.31.255.255. 172.32.x.x is public, so "anything starting with 172 is private" is wrong.
• 192.168.0.0/16 — 192.168.0.0 to 192.168.255.255, the range most home routers hand out.

Splitting into equal subnets

Lengthening the prefix by n bits splits a network into 2^n subnets. /24 to /26 borrows two bits and yields four; /27 yields eight. If you need six subnets, round up to the next power of two, eight, which means /27. The tool does the same rounding when you enter a desired count, and lists at most 4,096 subnets at once to keep the table usable.

Splitting costs addresses. Four /26s give 4 × 62 = 248 usable hosts, six fewer than the 254 of the original /24, because every subnet spends two addresses on its own network and broadcast.

Subnet Visualizer mode: mixed sizes with VLSM

Real networks rarely want equal pieces. The sales floor might need 100 hosts and the server room 20; carving a /24 into four equal /26s leaves sales short and the server room with room to spare. Variable-length subnet masking picks, for each request, the smallest block that fits, then places blocks from largest to smallest. Order matters because a block can only start at a multiple of its own size (a /26 at .0, .64, .128 or .192), so placing small blocks first tends to leave alignment gaps.

The Visualizer sorts requests largest-first by default; switch that off to place them in the order you typed. A request that does not fit in the parent network is flagged while the others are still shown. Count the gateway as a host: 30 PCs plus a router interface is 31.

In IPv6, /64 is the standard LAN

The tool is IPv4-only, and IPv6 networks seldom need host-count arithmetic anyway. RFC 4291 fixes the interface identifier at 64 bits for most unicast addresses, and stateless address autoconfiguration (SLAAC) assumes a /64 per link, so a LAN simply gets a /64. That single subnet holds 2^64 addresses, which removes the IPv4 instinct to squeeze subnets smaller to save space. For router-to-router links, RFC 6164 recommends /127, the IPv6 counterpart of the IPv4 /31.

Worked examples

How to use it

1. Enter CIDR notation (192.168.1.10/24) or an address plus netmask.
2. Read the network, broadcast and host range; enter a new prefix or a count to split it.
3. For subnets of different sizes, open the Subnet Visualizer tab and list the hosts each one needs.

Frequently asked questions

Notes

In VLSM mode the subnet type defaults to LAN (minimum /30), so even a request for 1 or 2 hosts gets a /30. For router-to-router links or loopbacks, switch to Point-to-point (allow /31 and /32); then 2 hosts get a /31 and 1 host a /32. For an ordinary LAN that also needs a gateway, count the gateway in the host number.