The Internet

Seth Breidbart once described The Internet as


 *  "the largest equivalence class in the reflexive, transitive, symmetric closure of the relationship 'can be reached by an IP packet from'." 

While most people don't look at it from a mathematic perspective, The Internet is, in practical terms, the collection of tier 1 networks which peer with one another and comprise the default-free zone (or 'DFZ'), and the waterfall of networks which peer with them or purchase transit to them; in short, if you pitch a packet over the wall with a public IP address as its destination, and you are connected to The Internet somehow, that packet should get there, and reply packets should get back to you.