One Location, One Zip Code, One Network Number

One Location, One Zip Code, One Network Number

So far, you learned what IP addresses look like when written down4 decimal numbers, between 0 and 255 inclusive, separated by periods. However, you also need to know more about the structure and meaning of these addresses. The structure and meaning of the addresses tells you about how IP addresses are used in an internetwork.

More postal service analogies can help, mainly because the concepts happen to be similar. In addition to the number, street, town, and state, the postal service uses zip codes to make it easy to sort the mail. (For anyone not familiar with zip codes in the U.S., a zip code is the U.S. version of what other countries call postal codes.) The postal service includes a zip code in everyone's address. It also ensures that everyone in the same general geographical area has the same zip code. For instance, everyone who lives in the same town that I do has the same zip code.

Zip codes allow the postal service to easily sort mail using automated mechanical sorting machines. The sorting machines look for the zip code on the front of the letter and ignore the rest of the address. Early technology for looking at and recognizing hand-written numbers and letters worked a lot better when looking for numbers, so the sorting machines were designed to look for the numeric zip code. Also, by sorting based on zip code, the USPS could overcome issues such as misspelled city names. So, the sorters could sort based on zip code, and the postal workers could deliver the letters for a particular zip code to the post office that handles mail for that zip code.

The only time the whole postal address is needed is when the letter gets to the final post officethe one that handles mail for that zip code. For instance, imagine there's a letter in the post office in Cincinnati, with a delivery address in Mason, Ohio, which is a suburb of Cincinnati, zip code 45040. The sorting machine in Cincinnati just looks at the zip code, which sorts the letter into the bin of letters that will be sent to Mason. When the letter is in the Mason post office, the local postal worker looks at the whole address, including the street name and number, so that the letter is given to the right mail carrierthe one who drives past that address each day. Figure 10-5 depicts this simple example.