Sending IP range not in same range as Ethx

[Dod]

Hello,

  4.1-ESV-R4 on Debian

  I have a problem setting subnet that is not part of the IP range on
  which the Eth of the machine running DHCP daemon is.

  My need :

  Send IP within 10.19.1.1 to 10.19.13.254
  mask 255.255.240.0
  gateway 10.19.0.1

  Problem  is that the Eth1 on wich the dhcp machine is running is not
  in  10.19  range  so  when  I  start  it  it  say it cannot find any
  interface with that subnet.

  Why  do  it  need  to be in same subnet ? is it for DHCP negociation
  when  it  tries  to  echo  the  IP  it  will propose to see if it is
  actually used ?

  Second question, what is the parameter that permit to set 10.19.0.1
  as gateway ?

  regards