Syntax for include files in /etc/network/interfaces

Solution 1:

from man 5 interfaces KNOWN BUGS/LIMITATIONS section:

It is not currently possible to divide up /etc/network/interfaces into multiple files. A feature that would make this possible is some sort of inclusion directive. No such feature exists in the current ifupdown program. For more information see Debian bug #159884.

UPDATE:

Since ubuntu 11.10:

Lines beginning with "source" are used to include stanzas from other files, so configuration can be split into many files. The word "source" is followed by the path of file to be sourced. Shell wildcards can be used. (See wordexp(3) for details.)

So the syntax is:

source path/to/configfile