🛠️ Tools
Small, useful things I got tired of Googling. Subnet maths, cron schedules, file permissions, your public IP, timestamps, Base64, a port reference and printable cheat sheets, all in one place and all running right here in your browser.
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Subnet Calculator Cron Generator What's My IP? chmod Calculator Timestamp Converter Base64 Encoder / Decoder Port Reference Cheat Sheets: chmod, umask, CIDRThose who understand binary, and those who don't.
🌐 Subnet Calculator
Type an IPv4 or IPv6 network in CIDR form (like 192.168.1.0/24). You get the netmask, the address range, how many hosts fit, and a binary view. You can also split it into smaller subnets, or check whether an address belongs to it.
- Each step down in prefix length doubles the size: a /24 has 256 addresses, a /23 has 512, a /16 has 65,536.
- Usable hosts = total addresses minus 2 (the network and broadcast addresses), except /31 (2, point-to-point) and /32 (1).
- Cloud providers reserve a few more addresses in every subnet (AWS and Azure reserve 5, Google Cloud reserves 4): check your provider's rules before you size a subnet tightly.
- Private IPv4 ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. IPv6 private (unique local) addresses start with fc or fd.
⏰ Cron Expression Generator
Build a crontab schedule from plain choices, or paste one you found and see what it really does. You get an English explanation, the next run times, and a ready-to-paste crontab line.
- Standard crontab has exactly 5 fields. Six or seven fields (with seconds or a year) belong to Quartz, Spring or AWS schedulers, not to Linux cron.
- If both day-of-month and day-of-week are set, cron runs the job when either matches, not both.
- Cron uses the server's time zone: check it with timedatectl. Kubernetes CronJobs use the same 5-field syntax (set spec.timeZone if you need a zone).
- Use absolute paths (cron's PATH is tiny), send output somewhere (>> file 2>&1), and escape a literal % as \% inside a crontab command.
- Stop overlapping runs with flock -n /tmp/job.lock command. List your jobs with crontab -l. Files in /etc/cron.d and /etc/crontab have an extra user field before the command.
0 0 * * * — midnight, when all the best outages start.
🕵️ What's My IP?
Shows the public IP address this website sees when you connect. That is the address the rest of the internet sees too.
- It is answered by this site's own server, with no outside service involved, and the address is not stored.
- If you use a VPN or proxy, you will see the VPN's address instead of your ISP's. On mobile networks many people share one address.
- Your IP address gives a rough region at best: it is not your street address.
- From a terminal: curl -4 https://techcommune.org/ip (IPv4) or curl -6 https://techcommune.org/ip (IPv6). To see your machine's local addresses, run ip -br addr.
🔐 chmod Calculator
Tick the permissions you want and get the octal number, the ls -l string and the chmod command. Or type a number (like 640) or a string (like rw-r-----) and watch the boxes change.
umask: what new files start with
- Common defaults: 644 for files, 755 for directories and scripts, 600 for anything private (SSH private keys must be 600, ~/.ssh must be 700).
- On a directory, "read" lists the names, "write" creates and deletes files inside, and "execute" lets you enter it. A directory without execute is a closed door.
- Set files and directories separately: find . -type d -exec chmod 755 {} + and find . -type f -exec chmod 644 {} +. Or use a capital X: chmod -R u=rwX,go=rX .
- Do not fix "permission denied" with 777. Fix the owner or group (chown, chgrp) or add the user to a group instead.
The 2038 problem: a future problem for a future you.
🕰️ Timestamp Converter
Turn a Unix timestamp (seconds, milliseconds, microseconds or nanoseconds) into a readable date, or a date into a timestamp. Paste a log line's number and see what time it really was.
Date to timestamp
- How the unit is guessed: about 10 digits is seconds, 13 is milliseconds, 16 is microseconds, 19 is nanoseconds. Change the unit if it guesses wrong.
- Always store and log times in UTC, and convert to local time only when showing them to people.
- A signed 32-bit timestamp runs out on 2038-01-19 03:14:07 UTC. Old 32-bit systems and file formats will wrap around to 1901.
- Handy commands: date +%s (now), date -d @1790000000 (timestamp to date), date -d "2026-09-29 14:30 UTC" +%s (date to timestamp).
🔤 Base64 Encoder / Decoder
Encode text or a file to Base64, or decode Base64 back to text or a file. It handles the URL-safe variant (used in JWTs and URLs) and Unicode text. Files never leave your browser.
- Base64 is not encryption. Anyone can decode it in a second. It only makes binary data safe to put in text (emails, JSON, URLs, config files).
- It makes data about 33% bigger: every 3 bytes become 4 characters.
- A JWT is three URL-safe Base64 parts separated by dots. Paste one part here in Decode mode to read it (do not paste real secrets into any website).
- On the command line: echo -n 'hello' | base64 and echo 'aGVsbG8=' | base64 -d. Use -n so a trailing newline is not encoded too.
Same data, slightly larger costume.
And the one you need is always the one the firewall blocked.
🔌 Port Reference
Look up what usually runs on a port, or which port a service uses. Search by number (443), name (ssh), a range (8000-8100) or a word (mail, redis, kubernetes). Click a port number for commands to check it on your own machine.
🔒 Encrypted 🔓 Clear text 🚫 Keep off the internet ⛔ Avoid "IANA" means the official registry lists this port for this service. "Convention" means people just use it that way.
The three port ranges
- 0 to 1023
- System (well-known) ports. On Linux only root, or a program with CAP_NET_BIND_SERVICE, can listen here (see net.ipv4.ip_unprivileged_port_start).
- 1024 to 49151
- Registered ports. Software vendors ask IANA to reserve a number, and anyone can listen here.
- 49152 to 65535
- Dynamic (private) ports, never assigned. Windows uses them for outgoing connections. Linux uses 32768 to 60999 by default: check with cat /proc/sys/net/ipv4/ip_local_port_range.
- A port number is only a convention. Anything can run on any port, so this list shows what is normally there, not what is there. To see what is really listening, run sudo ss -tulpn.
- TCP and UDP ports are separate: 53/tcp and 53/udp are two different doors that both happen to be DNS.
- Moving a service to another port (SSH on 2222, say) reduces log noise, not risk. Firewalls, keys and patches do the real work.
- A port can be open on the host and still blocked by the host firewall, a cloud security group, or a network firewall. Check every layer.
- Only scan systems you own or have permission to test. The full official list is the IANA "Service Name and Transport Protocol Port Number Registry" (iana.org).
chmod is where a bigger number means bigger trouble.
📋 Cheat Sheets: chmod, umask and CIDR
Quick-reference tables for the three things everyone forgets. Pick a sheet, or print all three (each starts on its own page).
chmod: permissions
Each octal digit is a sum: read 4 + write 2 + execute 1. Three digits are for the owner, the group and everyone else. So 754 is owner 7 (rwx), group 5 (r-x), others 4 (r--).
What r, w and x mean
| Bit | On a file | On a directory |
|---|---|---|
| r (4) | Read the contents | List the names inside (ls) |
| w (2) | Change the contents | Create, delete and rename entries inside (needs x as well) |
| x (1) | Run it as a program or script | Enter it (cd) and reach things inside by name |
To delete a file you need write and execute on its directory. The file's own permissions do not matter (unless the directory has the sticky bit).
Common modes (click one to load it into the chmod calculator above)
Special bits
| Bit | Octal | Symbolic | On a file | On a directory |
|---|---|---|---|---|
| setuid | 4000 | u+s | Runs with the owner's privileges (ignored on scripts) | No effect on Linux |
| setgid | 2000 | g+s | Runs with the group's privileges | New files inherit the directory's group |
| sticky | 1000 | +t | Ignored on Linux | Only a file's owner (or root) can delete or rename inside it, like /tmp |
Symbolic syntax: chmod [who][operator][permissions]
| Part | Values |
|---|---|
| Who | u owner g group o others a all. Leave it out and you get "all", filtered by your umask. |
| Operator | + add - remove = set exactly |
| Permissions | r w x X execute only for directories and files that already have it s setuid/setgid t sticky or u/g/o to copy that class's permissions |
| Command | What it does |
|---|---|
| chmod u+x script.sh | Make a script executable for its owner |
| chmod go-rwx file | Remove all access for group and others |
| chmod a=r file | Everyone read-only (nobody can write or execute) |
| chmod u=rw,go=r file | Same as 644 |
| chmod g=u file | Give the group the same permissions as the owner |
| chmod g+s dir | setgid on a directory: new files inherit the group |
| chmod +t dir | Sticky bit on a directory |
| chmod -R u=rwX,go=rX dir | Recursive: directories stay enterable, plain files do not gain execute |
| chmod --reference=a b | Copy the permissions of file a onto file b |
| find . -type f -exec chmod 644 {} + | Set files only (use -type d and 755 for directories) |
umask: the default permissions for new files
The umask lists the permission bits to take away from new files and directories. New files start at 666 (no execute), new directories at 777, and then the umask bits are cleared.
| Example | File | Directory |
|---|---|---|
| umask 022 | 666 minus 022 = 644 | 777 minus 022 = 755 |
Click a umask below to try it in the calculator above.
Where to set it
| Place | How |
|---|---|
| This shell | umask 027 to set it, umask to show it, umask -S for symbolic form |
| One user | A line in ~/.bashrc or ~/.profile |
| Everyone (login shells) | /etc/profile, a file in /etc/profile.d/, or /etc/bashrc on the RHEL family |
| Accounts and PAM | UMASK in /etc/login.defs (and pam_umask) |
| A systemd service | UMask=0027 in the [Service] section |
| A cron job | Cron uses its own default (often 022): put umask 027 && in front of the command |
| A running process | grep Umask /proc/PID/status |
- The umask only affects new files. It does not change files that already exist (that is chmod's job).
- Programs can override it on purpose: ssh-keygen creates private keys as 600 whatever your umask is.
- Many distributions use 022, and 002 for users who have their own private group. Hardening guides such as CIS commonly ask for 027 or stricter for users.
- Files never get execute from the umask: they start at 666, so scripts always need an explicit chmod +x.
CIDR: prefix lengths and subnet masks
- Prefix length = how many bits are the network part. A /24 keeps 24 bits for the network and leaves 8 for hosts.
- Addresses = 2(32 − prefix). Usable hosts = that minus 2 (network and broadcast), except /31 and /32.
- Each step in the prefix halves or doubles the size: /24 has 256 addresses, /25 has 128, /23 has 512.
- Block size trick: the size of a subnet in the changing octet is 256 minus the mask value there. A /26 has mask 192, so blocks of 64 (.0, .64, .128, .192).
- The interesting octet: /9 to /16 is the second, /17 to /24 the third, /25 to /32 the fourth.
IPv4 special and private ranges
IPv6 prefix lengths
IPv6 special ranges
Use the subnet calculator above to work out any specific network.
Right up until it fixes every security problem into existence.
Coming Soon
- More tools, whenever I catch myself Googling the same thing twice