OK, its a long story my friend

we have a firewall (Sonicwall) used as firewall and VPN site to site, and its offering DHCP inside the organisation, its connect to a switch (HP Procurve), there is a DSL modem offering internet connection through the switch and wireless connectivity, the DHCP server in Sonicwall give the users the gateway IP address. while the wireless users using the DSL modem as their gateway so the DHCP server in this DSL modem offer its IP address as gateway for the users, the problem is some users internally getting their DHCP settings from the DSL modem instead of the sonicwall.