Okay, here's how subnets work. It's not magic or rocket science, it's real simple.

Let's assume that you have a bunch of PCs all plugged in to a hub, or even a group of hubs and switches.

The PCs have to agree on an addressing scheme.

So you say "10.134.75.1" for the first computer, and "10.134.75.2" for the second computer, etc.

The subnet mask is the key. It says "listen to messages from all addresses with the bitmask of...", and of course you put in a bitmask of 255.255.255.0, which means that the computers all listen to messages from your address binary-ANDed with 255.255.255.0, which works out to: it listens to all addresses starting with 10.134.75.xxx.

There's nothing special about your local lan that says you can't have a thousand different little ad-hoc addressing schemes. The trick is that if you have a different address (say, 10.135.75.xxx), then no one on the 134 subnet can ping or communicate with your computer at all. But if they changed their address to be 135 then they could.

There's no special hardware or anything that says "traffic is only allowed on the 135 subnet. All other subnets are illegal." That's not how it works. You could have tons of traffic on other subnets and (here's the key) your PC merely doesn't bother listening to it. That's what a subnet is for, to filter the traffic. And the filtering is done at the PC, with the settings in the network control panel.

See, most people think of subnets as something physical involving hardware and cables. They're not. Subnets are just arbitrary choices of numbered addresses.

Now, I'm avoiding a discussion of how things like routers and gateways work. Because those don't apply to just "you and an empeg" plugged into the same hub/switch. Since that's what you're doing, we don't need to talk about how a router/gateway scheme allows multiple subnets to communicate with each other. But just know that if you have routers and gateways, they need to be specifically programmed to deal with certain subnets, and if you have a private little subnet you made for yourself, it will be completely ignored by any routers/gateways.

Anyway, back to what I was saying...

When you create another TCP entry in Windows, you're not messing up the network. You're simply saying "Hey, computer. I want you to listen for traffic coming from these addresses, too."

The only way it could be an issue is if someone else had also created an ad-hoc address group that matched yours, and then only if the individual addresses conflicted. And that's highly unlikely. And even if it did, you'd know instantly because you'd get an error message and then you could just pick a different digit.

Is the light beginning to turn on?