This is something I have to do with on a daily basis. The list sounds silly, but is spot on:

1. The network is reliable.
2. Latency is zero.
3. Bandwidth is infinite.
4. The network is secure.
5. Topology doesn't change.
6. There is one administrator.
7. Transport cost is zero.
8. The network is homogeneous.

usually we are affected by all of them