Why does my computer's IP address change?

Your router may sometimes change the addresses of computers on the network. This can happen because the router dynamically assigns addresses via a service called DHCP. This guide details how you may give computers on your network a static IP address, one that will not be changed.

Find your MAC address. (Not as in Mac OS!)

A MAC address looks like ff:00:ff:00:ff:00 or FF-00-FF-00-FF-00. (The numbers/letters will probably be different.)

On Windows...

Run cmd.exe. Enter route print into the prompt.

Look for "IPv4 Route Table", below should be "Active Routes". Under the "Network Destination" column, find the entry. Your computer's IPv4 address is under the "Interface" column.

Enter ipconfig /all into the prompt. Look for lines starting with "IPv4 Address" or "IP Address", and compare the address on that line to your computer's IPv4 address. Once you've found a match, look in the same group of lines for the line starting with "Physical Address". Your computer's MAC address is on that line.

On Mac OSX...

Open the Terminal app. It should be under Utilities in your Applications directory. Enter route get default | grep interface into the prompt.

Now enter ifconfig, followed by a space and then the name of the interface found above. Look for the line starting with "ether", your computer's MAC address is on that line.

On linux...

Open a terminal and enter ip route show default.

In the column after "dev" is the default interface's name.

Enter ip addr show, followed by a space and then the name of the default interface. Look for the line with "link/ether" in the first column. Your computer's MAC address is in the next column.

Assign a static IP address via your router.

If you don't know how to access your router to configure it, you can learn here.

Navigate to DHCP, Gateway, or a similarly named page. You may have to navigate through several pages or tabs.

Now look for reservations, leases, or similarly named and add a new entry or client.

Set the MAC address to the MAC address of your computer. Set the IP address to your computer's currently assigned address. You may also set the IP address to an address between the DHCP start and end addresses.

Save or apply changes as many times as necessary.