libnsl library – libc no longer includes networking functions. libnsl must be explicitly specified at compile time to link the network services routines.
Old interfaces – Many old interfaces are supported in the libnsl library, but they work only with TCP or UDP transports. To make full use of new transports, you must use the new interfaces.
Name-to-address mapping – Transport independence requires opaque addressing. This requirement has implications for applications that interpret addresses.