Pipes.

[vladone]

Try this:
ipfw pipe 100 config bw 500kbit/s mask dst-ip 0x000000ff
ipfw add 200 pipe 100 ip from any to 10.0.0.0/24 out xmit em1
ipfw add 65000 allow ip from any to any

Is strange how is blocked network! Try to test your rules, beginning
from simply situation, using only few needed rules.
Queue about 50Kbytes, i think that is set by default, so no need to
set again.

-- 
Best regards,
 vladone                            mailto:vladone at spaingsm.com