The U.N.'s "Code for Trade and Transport Locations" [0] was new to me and its use makes sense in some cases. I was also quite surprised to see on the list two _very small_ towns where I have facilities.
For those who have no gear outside of the U.S., however, CLLI codes [1], common in the telco industry, might be a better fit. Some of the codes on the U.N. list aren't very "intuitive" and the CLLI codes (the six-character "Network Site Codes", specifically) make a bit more sense. The U.N. code for Albin, Wyoming, for example, is "US AQI" while the (geographical portion of the) CLLI code is ALBNWY.
Side note: As seen below, you'll often see CLLI codes in PTR RRs when running traceroutes across the Internet. This can sometimes help when troubleshooting networking issues ("LSANCA" refers to Los Angeles, California).
$ traceroute route-server.ip.att.net
traceroute to route-server.ip.att.net (12.0.1.28), 30 hops max, 60 byte packets
... snip ...
3 ge-0-7-0-24.r04.lsanca03.us.bb.gin.ntt.net (129.250.198.185) 0.691 ms 1.393 ms 1.098 ms