Why can't the IPv4 networking be built as a module?

Hi All,

Naive question of the day:

I'm trying to cut down the size of a 3.3 kernel and one thing I have
noticed is the TCP/IP v6 can be built as a module, but v4 cannot. Is
there any particular reason for this, or is it simple a case of nobody
has gone to the effort of modularising the v4 networking components?

Regards,

Graeme