system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Subnet masks. I just don't get it.
gpt
Imagine you send lots of letters between yourself and others around the apartment block, your city and the world. Your address could look like:
World.UK.London.120 Fleet Street.156

Let's say the postman is *really* lazy and doesn't want to waste time reading your full address every time; He suggests using a subnet mask. If you wanted to send a letter to someone else in your apartment you'd send to the address:  ...298. The postman assumes it's the same country, city, etc and delivers it to room 298 in the same apartment block.

If you had to send a letter outside your apartment block, you'd send the letter to the full address. World.UK.Manchester.1 Deansgate.1A and the postie would know to route it outside the apartment block.

Imagine sending millions of parcels every minute, using subnet masks can relieve traffic load and congestion :)