A bang path is an archaic form of addressing that was used in e-mail and newsgroup postings sent through the UUCP network. Unlike the Internet, with its distributed DNS allowing direct addressing of all hostnames throughout the network (which addresses can be looked up hierarchically), UUCP worked by systems passing messages to one of the usually small number of other systems directly known by the first system, which in turn passed them on to another system known to it, and so on (like a bucket brigade). Since there was no global routing table to tell systems what route to take to reach the final destination, the original sender had to specify a complete list of pass-through nodes. This was done in the form of a list of node names separated by the exclamation point (!), which was nicknamed "bang", giving the name "bang path".
A bang path might look like
idis!decvax!decwrl!rhea!node!user, where the final item is the username of the recipient and the preceding item is the hostname of the system where the mail is to be delivered.
Bang paths have gone out of use for addressing e-mail, in favor of domain-based addressing, but they are still used in the headers of Usenet messages to denote the path a particular message has traveled.