I feel like you were done dirty. When I was in grad school 12 years ago, our networking classes used mininet to simulate networks on a single host. It's mostly meant for developing SDN systems, but probably would have met your needs and supports way more.

On the other hand, building even a tiny subset but doing it yourself from scratch is a great way to learn. I made a very poor man's VM image builder for HyperV years back because Packer didn't have a builder for it at the time and that was a pretty interesting experience. Finally grokked the Windows object model and even though I still don't use it, I at least no longer jeer at PowerShell.

I'm interested in the answer to your question, too, but as a customer of an ISP. I don't work for one. I was the first owner of my house and when they hooked me into their network, whoever did messed up my neighbors badly, putting them on the wrong circuit and bleeding noise into adjacent neighborhoods. For three years, complaint calls would get our network cut by third-party contractors with no warning, then we'd have to call and get it reconnected. I don't know how they're supposed to do it, but know it can cause quite a mess when they do it wrong.