Here's an example of how houses are numbered in Venice. Looks simple enough, doesn't it? Here's the catch. If you are given someone's address, you get a sestiere ( district ) name and house number, for example San Marco 2231. You have no way of knowing WHERE in San Marco that might be, without also knowing the name of the calle or fondamenta this address is located at. Fortunately, this system only allows a house number to be used once in any given sestiere, unlike in the US where you might find a 10 Main Street and a 10 Elm Street. Here, 2231 could be found once in each district- that's it. The question is always where in that district??? I have no clue how the mail carriers do it. At first I thought the numbers must be sequential, but I have discovered that from one street to the next the numbers might be hundreds apart.
To help me locate places, I've discovered a website called VeniceExplorer which has a cool feature. You put in the Sestiere and number, it pin points the location on a map so you can see the street name. Without this I would be so lost!!!