Port numbers & dictionaries
Ports and services
Lesson 9 of 20 · 8 min · Module 3: Ports and endpoints
An IP address finds the computer; a port number finds the program on it. Web servers usually listen on 80 and 443, SSH on 22.
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By the end, you’ll be able to…
- Explain what a port identifies
- Look up values with dict.get
- Validate a numeric range
The idea
Port numbers range from 0 to 65535. Ports 0 to 1023 are well-known (reserved for standard services), 1024 to 49151 are registered, and 49152 to 65535 are dynamic: short-lived ports chosen by clients.
SERVICES = {22: "ssh", 443: "https"}
print(SERVICES.get(22, "unknown"))01Address plus port
192.168.1.10:22 means "the SSH program on 192.168.1.10". One server can run a web server, a database, and SSH at once, each on its own port.
02Defaults avoid KeyError
SERVICES[8081] raises KeyError, while SERVICES.get(8081, "unknown") returns a safe default. Choose get when a missing key is normal.
SERVICES = {22: "ssh"}
print(SERVICES.get(8081, "unknown"))unknown03Ports are 16-bit numbers
16 bits give 65,536 values, 0 to 65535. Anything else cannot be a port, so raising ValueError stops bad data early.
Your turn
service_for(port) that returns the service name for 22 (ssh), 25 (smtp), 53 (dns), 80 (http), 443 (https), 3306 (mysql), and 5432 (postgresql), and "unknown" otherwise. Write port_range(port) that returns "well-known", "registered", or "dynamic", and raises ValueError for numbers outside 0 to 65535.What the checks look for
- Names common services
- Returns unknown for other ports
- Classifies port ranges at their boundaries
- Rejects ports outside 0–65535
Take this with you
The address finds the machine; the port finds the program.
Go a little further
Add a reverse lookup, port_for(service), that returns the port for a service name.