Does email coming from the same IP address imply that it was the same computer?

In the simplest case, an IP address uniquely identifies your computer on the internet. However for many reasons that's becoming less and less common as computers proliferate.

Routers

An IP address only identifies whatever it is you have connected to the internet. In many cases these days, that's a router:

In a case such as this, all the computers to the left of the router will appear on the internet as having the same IP address. That IP address is actually assigned to the router, and it handles routing the traffic to the appropriate computer on the local network.

In a case like this the IP address you've extracted from your email headers may get you as far as the router, but that's it. You can't tell which computer behind the router was responsible for it.

The diagram above is a common home or small business configuration. It's important to realize, though, that in larger installations there could easily be hundreds of computers sharing a single or smaller set of IP addresses. Once again, with just the internet IP address, there's no way to tell which computer sent your email.

Dynamic IP Addresses

Many computers are connected to the internet using what's called a "dynamic" IP address. The IP address is assigned to that computer when it first connects to the internet, and is released when it disconnects. A common example is dial-up connectivity where the connection and disconnection are both obvious and frequent. Persistent connections can also use dynamic IP addresses, and in fact can be re-assigned a new address even without having to disconnect - though typically that's not the case. However even the slightest disconnection could cause a new IP address to be assigned.

What's important to note here is that the IP address you were assigned yesterday might very well be used by someone else today.

That means if your sender is using a dynamic IP address, then it might be someone else entirely if you see that same IP address in another email at a later time. There's no obvious way to know.

Local IP addresses

If the address you see begins with 192.168., 172.16. through 172.32. or 10. then it's not an internet IP address at all, but rather a local IP address assigned by a router.

Looking at the diagram above again, you can see that internet IP addresses are assigned to the router's connection to the internet. However on the left, on the local side of the router, the addresses are assigned from a range of IPs reserved for local networks. Most home and small business routers assign from the 192.168. address range.

The problem here is that if that's the IP address you're seeing, then it tells you pretty much nothing. There are probably tens of thousands of machines with that 192.168.?.? IP address, scattered on local networks around the internet.

Without the internet IP address, there's just no way to get closer.

The Bottom Line

Ultimately, as I've said time and time again, trying to use IP addresses to locate someone is futile for the average person. Yes, technically there may be ways to backtrack, but it's complex, and often involves breaching privacy barriers that will require law enforcement and/or court orders.