The WKS record type was, at one point, a record listing all network services available on a host. It consisted of a bitmap where each bit represented a port number, counting from 0 (!), where a bit being set indicated that the host offered a service on that port. This scheme only worked for low port numbers; it also inappropriately conflated DNS names with hosts. (The latter was perhaps a reasonable assumption when WKS was created sometime in the '70s or '80s, but it certainly isn't anymore.)

The most definitive references to WKS are probably:

* RFC 1035 (1987), which defines the record format.

* RFC 1912 (1996), which noted that "[WKS records] serve no known useful function, except internally among LISP machines. Don't use them."