akinomatics
Which contrib can I download to get the filtration done or how can i get the to do custom fragment you mentioned.
I am not aware of the need for a custom template.
I do what you want in server manager Hostnames & addresses panel.
Add the workststion name, the mac address (obtained previously from the workstation NIC) & then the LAN IP you want DHCP to allocate to that mac/workstation (or device).
Usually I run the admin console first & in the Configure this server screens, I narrow down the DHCP range (of IPs that are automatically allocated by the system when not forcing the IP).
As Stefano says, the workstations IPs you want to forcibly allocate by mac address, must be outside of the DHCP auto allocate range.
Remember if you change NICs or workstation hardware, then you need to reconfigure the mac address & IP in the Hostnames panel (ie delete old, add new).
This functionality has been part of sme server since very early (eg v3.x).