You can assign certain hosts always the same IP address via dhcp. The FreifunkFirmware (FF) uses dnsmasq as an DNS and DHCP Server. First add a IP, Hostname entry to /etc/local.hosts. Secondly you have to bind the ethernet address of your machine to the name just added to local.hosts. Third and last Step is to restart dnsmasq.
Use ssh to login to your Freifunk Device.
Add IP and Hostname to local.hosts
root@gw3-wrt54g:~# rm /etc/local.hosts
root@gw3-wrt54g:~# cp /rom/etc/local.hosts /etc/
root@gw3-wrt54g:~# vi /etc/local.hosts
Add Ethernet adress and Hostname to ethers
root@gw3-wrt54g:~# rm /etc/ethers
root@gw3-wrt54g:~# cp /rom/etc/ethers /etc/
root@gw3-wrt54g:~# vi /etc/ethers
Restart dnsmasq
root@gw3-wrt54g:~# /etc/init.d/S50dnsmasq stop
root@gw3-wrt54g:~# /etc/init.d/S50dnsmasq start
Everytime the machine with the Ethernet address entered in ethers is requesting an IP address, the IP assingned to the requesting hostname in local.hosts is awnsered.