Your computer’s IP address wasn’t “changed” so much as it was broken.
If your computer ends up with an IP address beginning with 169.254, then something is definitely wrong.
Where IP addresses come from
Typically, the IP address of your computer is assigned when you boot your machine: using DHCP, your computer asks for an IP address.
If you are connected to a NAT router, it’ll be the one acting as a DHCP server and handing out IP addresses. In cases like this, you’ll most commonly get an address in the 192.168.x.x range.
If you’re directly connected to the internet, your IP address could be almost anything, but it’ll be assigned by a DHCP server run by your ISP.
IP addresses beginning with 169.254, however, are special.
When there’s no answer
This difficulty arises when your computer asks for an IP address, and no one responds to the request.
Whether due to a network problem, maybe not being on a network at all, or perhaps there’s no DHCP server to hand out IP addresses, the result is the same: the request for an IP address assignment goes unanswered.
Your machine waits for a while, and then gives up.
But when it gives up, it invokes what’s called Automatic Private IP Addressing, or APIPA, and makes up its own IP address. And those “made up” IP addresses all begin with 169.254.
Almost as good as no IP at all
The problem is that while making up an IP address actually does solve a few obscure problems, they’re all problems you and I don’t really care about as we try to use our computers normally. A 169.254 IP address is pretty close to useless.
The reason that IP addresses are assigned in the first place is so the networking equipment knows where to send or how to route packets that are destined to reach your machine at a particular address. In other words, when things work, the networking equipment knows your machine has this address. If your machine makes up its own address, no one else knows. As a result, there’s no way to know how to get to your machine.
It’s like having an unlisted phone number that’s so unlisted, even the phone company doesn’t know it. No one can call you.
Getting a 169.254 address simply tells you that the machine cannot reach the DHCP server over the network.
Something isn’t working, but exactly what is difficult to say.
- If you’re connected via a NAT router, then either your computer is not actually connected on the network to that router, or the router itself isn’t responding to the DHCP request. I suggest triple-checking your cabling and wireless settings, and perhaps rebooting the router and/or wireless access point. If you have a wireless connection, sometimes simply disabling and re-enabling it can resolve some situations, as might rebooting your computer.
- If you’re connected to the internet directly, then there’s a problem with that connection. Check the cabling to your broadband modem, and check the cabling from your broadband modem to your telephone or cable line. And, if all else fails, call your ISP – it may actually be their DHCP server or some other item they control that’s at fault.
Bottom line: if you’re getting a 169.254 IP address, something about your network setup isn’t working, and you probably want to investigate what and why.