no, if you have DHCP enabled then you can NOT use addresses in the range of 192.168.1.100/255 , in addition, with DHCP disabled you still can not use 192.168.1.255 because that address is reserved. the address you have assigned (192.168.1.21) is fine. so for example, you said that you want to allow ssh connections to 192.168.1.21 from out side of your LAN. you would do this with port forwording, you set up on the router to forword port 22 to 192.168.1.21. then to access your ssh server from out side of your lan all you have to do is connect using your external ip address (what your isp assigns). you can also set up DNS with www.no-ip.com like you said.