Устанавливаем сервер с портов

deathstar# cd /usr/ports/net/isc-dhcp41-server
deathstar# make install clean

И создаем конфиг (у меня он выглядит так):
deathstar# ee /usr/local/etc/dhcp.conf
## домен
option domain-name ruscoolection;
## выдаваемые ДНС сервера
option domain-name-servers 8.8.8.8 , 8.8.4.4;
## шлюз
option routers 192.168.255.100;
authoritative;
log-facility local7;
## диапазон IP которые выдавать,и маска подсети
subnet 192.168.255.0 netmask 255.255.255.0 {range 192.168.255.0 192.168.255.255;}
## Закрепленные IP по Mac адресу сетевых карт моего ПК и нетбука
host sysadmin {hardware ethernet 00:18:F3:E4:C0:C4; fixed-address 192.168.255.103;}
host eeepc {hardware ethernet F4:6D:04:26:A8:C3; fixed-address 192.168.255.250;}

В /etc/rc.conf добавляем
dhcpd_enable="YES"
dhcpd_conf="/usr/local/etc/dhcpd.conf"
## на каком интерфейсе будет слушать
dhcpd_ifaces="fxp1"
dhcpd_withumask="022"
dhcpd_devfs_enable="YES"
dhcpd_rootdir="/var/db/dhcpd"

И стартуем
deathstar# /usr/local/etc/rc.d/isc-dhcpd start
Starting dhcpd.
Internet Systems Consortium DHCP Server 4.2.1-P1
Copyright 2004-2011 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Wrote 0 deleted host decls to leases file.
Wrote 0 new dynamic host decls to leases file.
Wrote 5 leases to leases file.
Listening on BPF/fxp1/00:e0:18:5a:ca:72/192.168.255.0/24
Sending on BPF/fxp1/00:e0:18:5a:ca:72/192.168.255.0/24
Sending on Socket/fallback/fallback-net
deathstar#

Как видите — ничего сложного нет.