Router configuration with Linux

Most routers must be configured before usage. Unfortunately the manual describes the procedures only for Windows and sometimes for Mac OS. The configuration is often don through a web front end of the router. Often these are easily reached. Some products however are protected more strongly (Wireless router and access points). Here the access is only allowed from one IP address, which is not likely to be distributed by the DHCP. Therefore the IP of the network interface has to be changed to be able to connect to the web front end.
This command does the trick (changes the IP to

ifconfig wlan0 netmask up