Linux chmod Permissions & IPv4 CIDR Subnet Mask Calculator

chmod Permissions and CIDR Subnets: The Numbers, Finally Clear

Two of the most copy-pasted numbers in tech are chmod modes and subnet masks, and both are just compact encodings of simple ideas. A chmod value like 755 is three octal digits — owner, group, others — where each digit adds read (4), write (2), and execute (1). So 7 means read+write+execute, 5 means read+execute, and 755 gives the owner full control while everyone else can read and run but not modify. The classic 644 for files and 755 for directories is not superstition; it is the least privilege that keeps websites working.

CIDR notation compresses the same kind of thinking into a slash and a number. A /24 means the first 24 bits of the address are the network, leaving 8 bits for hosts: 2^8 = 256 total addresses, minus network and broadcast, giving 254 usable. A /16 holds 65,536 addresses; a /28 holds 16, with 14 usable. The pattern is always total = 2^(32 − prefix), usable = total − 2. Once that clicks, reading "10.0.4.0/22" tells you instantly that the range spans 10.0.4.0 through 10.0.7.255 — four full /24 blocks, 1,022 usable hosts.

The mistakes that bite are always the same ones. Someone chmods 777 "to make it work" and hands every user on the system write access to their web root — the number one permission sin in shared hosting. Or a developer carves a VPC into /28s and discovers too late that cloud providers reserve several addresses per subnet, leaving almost nothing usable. This calculator shows the permission bits spelled out in plain words and the subnet math with network, broadcast, range, and usable counts, so you stop guessing and start knowing.