Theres a number of DNS seed servers that are hard coded into the Bitcoin core codebase. A Bitcoin client does a nslookup on these seeds to find intial peers to connect to and bootstraps this way.

For a bit more detail and if you're interested in this sort of thing, have a read of my blog article I wrote about the technical side of Bitcoin: http://www.samlewis.me/2017/06/a-peek-under-bitcoins-hood/

The original post and yours complement each other very well, thanks for the information!

Hi! Author here. Thanks for the appreciation!

You're right, I didn't describe the details of networking. My main goal was to keep it as short as possible while still explaining the main "pillars" of Bitcoin in detail, and networking did not make the cut. As far as my understanding goes, there isn't anything inherent to Bitcoin in the way its network works (it could apply to most other distributed networks), so I thought including this would stretch things too much and stray too far off topic in a way.