From the client side, apt-cacher-ng can be used as drop-in replacement for apt-cacher. The same rules apply, e.g. Debian/Ubuntu users should EITHER:
Acquire::http { Proxy "http://CacheServerIp:3142"; };
OR:
deb http://ftp.uni-kl.de/debian etch main
now would become:
deb http://192.168.0.17/ftp.uni-kl.de/debian etch main
(assuming that CacheServerIp is 192.168.0.17).
Mixing both configuration methods is not recommended and will lead to obscure APT failures in most cases.
Additionaly, leading path component containing "apt-cacher/" or "apt-cacher?/" might be ignored by the server during the URL processing. This is intended behavior and exists to maintain backwards compatibility to sources.list entries configured for early versions of Apt-Cacher (based on CGI technology).