What is CLLI?

A CLLI (Common Language Location Identifier) code is a short telecom code identifying a building, such as a carrier exchange, by city, state and site.

CLLI codes were created in the North American Bell System and are now maintained as a commercial standard used mostly by carriers in the United States and Canada. The full code is up to 11 characters, built from fixed-width parts. The first four letters abbreviate the town or city, the next two give the state or province, and the next two identify a specific building, giving an 8-character site code. An optional final three characters identify a particular piece of equipment or entity in that building. So a code beginning CHCGIL points at Chicago, Illinois, with the following two characters saying which building. Because the format is so compact and consistent, it suits machine-friendly names and spreadsheets of circuits and sites.

Network engineers mostly meet CLLI codes inside router hostnames, especially in the reverse DNS names that traceroute prints next to each hop. A name like ae2.cr1.chcgil01.example.net hints that the router is a core router in a Chicago building, which helps when working out where along a path latency jumps or packets die. Other carriers use IATA airport codes (for example ord for Chicago) or their own abbreviations instead, and some use none at all. Treat any location in a hostname as a hint rather than proof: names are just PTR records chosen by the operator, and they can be out of date after equipment moves. The Linux Network Connectivity lesson shows where those traceroute names come from.