# Is It the Network? ping, curl -I and Ports When a website or server doesn't answer, four quick checks find the broken link: ping asks whether the computer answers, curl -I whether the website does, ss or lsof which program is listening on a port, and dig which address a name points to. Journeyman · Unix · Linux · Updated November 10, 2026 ## After this lesson you can - Check whether a computer answers with ping, and whether a website answers with curl -I - Find what is listening on a port with ss or lsof, and what address a name points to with dig ## Key takeaways - ping -c 3 host asks a computer to answer three times. Replies mean it's on and reachable. Some servers ignore ping, so silence alone proves nothing. - curl -I asks a website for its headers only. The first line's status code, such as 200 or 404, says whether the site itself works. - A port is a numbered door on a computer, such as 22 for SSH and 443 for websites. ss -tln, or lsof on a Mac, shows which doors a program is listening at. - dig +short name prints the IP address a name points to. Nothing back means the name doesn't exist, or doesn't point anywhere yet. A book club member texts Sam: "The website's down!" Is it their Wi-Fi, the club's web address, the server, or the program on the server? Each needs a different fix, and guessing wastes an evening. Four one-line checks find which link in the chain is broken: does the computer answer, does the website answer, is the program listening, and does the name point to the right place. Help-desk techs and administrators run checks like these every day, and web developers reach for them whenever a site works on their own computer and nowhere else. To practice on a site you control, serve the book club's pages from *Copying Between Computers: scp and rsync* with Python's small built-in web server. Open a second terminal window and type: ```bash cd ~/terminal-practice/book-club/site python3 -m http.server 8000 ``` It keeps running in that window and serves the site at `http://localhost:8000` until you press Ctrl+C there. (On a Mac, the first `python3` may offer to install Apple's command line tools; say yes.) Type the rest in your first window. ## Does the computer answer? ping `ping` sends a computer a tiny "are you there?" and times each answer. `-c 3` stops after three; without it, ping goes on until you press Ctrl+C: ```bash ping -c 3 example.com ``` ```text PING example.com (104.20.23.154) 56(84) bytes of data. 64 bytes from 104.20.23.154: icmp_seq=1 ttl=55 time=15.0 ms 64 bytes from 104.20.23.154: icmp_seq=2 ttl=55 time=16.1 ms 64 bytes from 104.20.23.154: icmp_seq=3 ttl=55 time=15.9 ms --- example.com ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2003ms rtt min/avg/max/mdev = 14.960/15.652/16.058/0.491 ms ``` `104.20.23.154` is the computer's **IP address**, its number on the internet. Each reply took about 15 milliseconds (thousandths of a second), and none were lost: that computer is on, and the way to it works. Your numbers will differ. A name that doesn't exist fails before anything is sent: `ping: bookclub.invalid: Name or service not known` (a Mac words it differently). One catch: some servers ignore ping on purpose, so no replies doesn't prove a server is down. Go on to the next check. ## Does the website answer? curl -I `curl -I` (a capital I) asks a website for its **headers** only, the short lines that come before a page, and the first line holds the status code from *Talking to the Web: curl*. `head -n 1` keeps that line: ```bash curl -sI https://example.com | head -n 1 curl -sI http://localhost:8000/ | head -n 1 curl -sI http://localhost:8000/menu.html | head -n 1 ``` ```text HTTP/2 200 HTTP/1.0 200 OK HTTP/1.0 404 File not found ``` 200 means the website works; 404 means the website works but that page isn't there, so check the address or the deploy. If ping answers and curl doesn't, the computer is fine and the problem is the website program on it. ## Is the program listening? Ports One computer runs many programs that talk to the network, so its IP address is like an apartment building's street address, and each program waits behind its own numbered door: a **port**. SSH listens at port 22, websites at 80 (`http`) and 443 (`https`), and your practice site at 8000. ![An apartment building labeled with the street address 104.20.23.154, the computer's IP address, with four numbered doors: 22 with SSH behind it, 80 with a website (http), 443 with a website (https) and 8000 with the practice site](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/11/a4a22f3dd5654d65a8241a41fcad3a95.webp) `ss -tln` lists the doors that programs are listening at (`-t` TCP, the kind websites and SSH use; `-l` listening; `-n` numbers instead of names). Ask about one port: ```bash ss -tln 'sport = :8000' ``` ```text State Recv-Q Send-Q Local Address:Port Peer Address:Port LISTEN 0 5 0.0.0.0:8000 0.0.0.0:* ``` `0.0.0.0:8000` reads "listening at door 8000, on every address this computer has". `lsof -i` names the program behind the door, with its process number (*What's Running: ps, top and htop*): ```bash lsof -i :8000 ``` ```text COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME python3 2572 sam 3u IPv4 1859819 0t0 TCP *:8000 (LISTEN) ``` Now press Ctrl+C in the second window, and ask again: ```bash curl -sSI http://localhost:8000/ ss -tln 'sport = :8000' ``` ```text curl: (7) Failed to connect to localhost port 8000 after 0 ms: Could not connect to server State Recv-Q Send-Q Local Address:Port Peer Address:Port ``` Nothing listens, so nothing answers: the program stopped. On a server, `ss -tln` shows at a glance whether the web server is at 80 and 443 and SSH at 22. A Mac has no `ss`; `lsof -iTCP:8000 -sTCP:LISTEN` works on both. `lsof` shows other users' programs only with `sudo`. ## Where does the name point? dig Computers find each other by number; **DNS**, the internet's address book, turns a **domain name** such as `example.com` into an IP address. `dig +short` looks a name up and prints only the answer: ```bash dig +short example.com dig +short bookclub.invalid ``` ```text 172.66.147.243 104.20.23.154 ``` `example.com` points at two addresses, and `bookclub.invalid` at none, so the second line prints nothing. When a new site's name was set up today, or moved to a new server, `dig` tells you whether the name already points at the new address. If Ubuntu says `dig: command not found`, install it with `sudo apt install bind9-dnsutils`; a Mac has it already. | What you saw | Where it breaks | |---|---| | `dig` prints nothing | The name: it doesn't exist, or doesn't point anywhere yet | | `ping` fails, `curl` fails | The computer, or the way to it | | `ping` answers, `curl` can't connect | The website program, or its port: check `ss -tln` on the server | | `curl` shows 404 | The page: check the address, or what the deploy sent | ## Check yourself 2 questions to try on the lesson page. ## So what "It's down" can mean four different things. Four short checks, from the name to the page, tell you which one, so you fix the right thing first. ## Key terms - [ping](https://www.ask-codey.com/glossary/ping) - [IP Address](https://www.ask-codey.com/glossary/ip-address) - [Port](https://www.ask-codey.com/glossary/port) - [Domain Name](https://www.ask-codey.com/glossary/domain-name) - [DNS](https://www.ask-codey.com/glossary/dns) - [dig](https://www.ask-codey.com/glossary/dig) ## Cheat sheets - [Is It the Network? ping, curl -I and Ports](https://www.ask-codey.com/cheat-sheets/bash-network-checks-cheat-sheet) ## Related - [Talking to the Web: curl](https://www.ask-codey.com/lessons/bash-curl) - [Logging In to Another Computer: ssh](https://www.ask-codey.com/lessons/bash-ssh) - [Running Commands on Another Computer](https://www.ask-codey.com/lessons/bash-remote-commands) Source: https://www.ask-codey.com/lessons/bash-network-checks