I went to the Columbus campus of Ohio State, whose zip code is 43210. The company I work for now has a nearby plant whose zip code is 12345.
Zip codes generally increase from east to west. Because the lowest zip codes are in the Northeast, you might expect the highest ones to be in the Southwest, but they're actually in the Northwest. San Francisco (94xxx) has higher zip codes than Los Angeles (90xxx-91xxx), and Alaska (995xx-999xx) has the highest of all. Hawaii (96xxx) has higher zip codes than San Francisco, but lower ones than Seattle (981xx). Strangely, the lowest zip code on the West Coast of the continental US, 90001, is in Los Angeles, which is neither the northernmost, southernmost, easternmost or westernmost place in California. (The lowest overall zip code is 00501, which belongs to an IRS office in Holtsville, NY.)
Another exception to the geographic trends is the use of zip codes for Navy ships, which have no fixed location. US territories have zip codes, too. Some remote, uninhabited Pacific islands have zip code 96898, which has the "preferred" postal name of Wake Island, HI.
Because zip code prefixes are assigned by state, the two adjoining zip codes that differ by the largest number must be along a state boundary. Most likely they along the border between Tennessee (370xx-385xx) and Arkansas (716xx-729xx). If you figure out which two they are, drop me a line. I tried Googling "two zip codes biggest difference" but to no avail. If someone Googles that in the future, it'll just bring them to this page.
According to the Post Office, there are about 42,000 zip codes. Since there are 100,000 five-digit numbers, that means less than half the available numbers have been assigned.
I wondered about pairs of cities whose zip codes are the reverse of each other. Here's a list of some places I've lived along with the city that has the reverse zip code.
Perry, OH 44081 ---- Easton, PA 18044
Cleveland, OH 44106 ---- Kaneville, IL 60144
Columbus, OH 43235 ---- Milwaukee, WI 53234
Fremont, CA 94538 ---- Riggins, ID 83549
Lakewood, CA 90713 ---- Americus, GA 31709
Niskayuna, NY 12309 ---- 90321 is not the zip code of any place
Westlake, OH 44145 ---- 54144 is not the zip code of any place
Spotsylvania, VA 22551 ----- Bedford, PA 15522
I've never been to any of the "reversed" places. Someone with a lot of free time should write a script where you enter all the cities you've been to, and it figures out whether you've ever been to two cities whose zip codes are the reverse of each other.
Of course, some zip codes read the same forward and backward. If you've been to Romulus, NY (14541), Lanett, AL (36863), or Kettleman City, CA (93239) you've covered both the forward and backward version of those zip codes. I missed a palindromic zip code by just one digit when I lived in Westlake, OH. 44144 belongs to Brooklyn, OH.
In some areas, you can see that the zip codes were assigned alphabetically. For example,
44077 Painesville, OH
44084 Rock Creek
Those places are all within 30 miles of each other. If anyone ever founds a Quincy, OH in the northeast corner of the state, its zip code will undoubtedly be 44083. I wonder why they left two unassigned numbers between Painesville and Parkman. Was the Post Office following a consistent rule based on the frequency of the second letter, or was it just an oversight by a bored employee in a dusty government office back in 1963 when all the assignments were being made? Or were 44078 and 44079 formerly assigned to post offices that have been closed?
Only three zip codes in the whole US are counting sequences of numbers. One is 12345, which I mentioned already; the other two are
23456 Virginia Beach, VA
45678 Scottown, OH
None of the others are assigned. I was thinking Boston, MA was 01234, but then when I sang the theme song to Zoom to myself, I remembered it is actually 02134.
Likewise, there are just three reverse counting sequences. They are 43210 (already mentioned) and
Killeen, TX 76543
Spaceport City, NM 87654 (This one kills me. It's not a real city, but a boondoggle state-funded spaceport that almost nobody uses. Similarly, the telephone area code for Cape Canaveral and Kennedy Space Center is 321. Get it?)
Then there's the totally awesome repeating-digit zip code. There are only 10 possible and of those 10 just one has been assigned. Drumroll, please...
Newton Falls, OH 44444
There was a big tornado there in 1985. They say the wind blew so hard, it jumbled up the numbers in the zip code...