So here's the deal. My router assigns ip addresses starting from 192.168.0.50. I have a couple of computers running ubuntu which i want the router to assign 192.168.0.50 and 192.168.0.51 every time they request for an ip address. Any ideas? Thanks in advance.:)
You have a few options.
1) DHCP reservations on your router. Some routers have this look around in the settings.
2) I'm pretty sure if you go to networking manager you can set a manual ip, but still be on DHCP. I mostly use cli.
3) Set a staic ip with this guide http://www.howtogeek.com/howto/ubuntu/change-ubuntu-server-from-dhcp-to-a-static-ip-address/
The simplest thing to do is to set computer A to a static IP of 192.168.0.50 and computer B to a static IP of 192.168.0.51 and configure your router to send DHCP addresses starting at 192.168.0.52 instead of 192.168.0.50. You may also wish to reserve addresses as outlined by wlraider70
using the linux program dhclient.
If you don't have linux installed, you can use a bootable Linux live CD.
 If you do this trick to create an address reservation for an actual Windows machine, first switch the NIC in Windows to a static IP, because after a reboot in DHCP mode Windows may send a DHCP request with its last used IP, which would annihilate your efforts with dhclient below...
The trick is to send a DHCP request with your desired IP address from the same NIC (or better: same MAC address).
Edit the /etc/dhcp/dhclient.conf and add the following line (you might have to first copy the file to a writeable location if booting from a CD):
Then stop and start dhclient as user root, where your NIC is eth0 (check by issuing ip addr show beforehand)
If successfull, your DHCP server will fulfill your prepared request. Sometimes you will have to delete the device entry in the DHCP table before issuing dhclient -4 -d ....
Then make that device IP address in the DHCP table of the router a fix address reservation. This will associate the IP to the MAC of the device and return the same IP everytime a DHCP request with that MAC address is encountered in the future.