The <tt>net</tt> directory holds the network functionality of FreeBSD that
deals with routing and generic interfaces: SLIP, PPP, Ethernet, packet
filters.

The 4.3BSD Internals book by Leffler, McKusick, Karels and Quarterman
covers <tt>kern</tt> in:

<dl>
<dt> <b>Chapter 11</b>, pp 311-341 <i>Network Communication</i>.
</dl>
<p>