The internet is made up of tens of thousands of separate networks, each independently operated and connected to the others. Each of those networks is called an autonomous system. 

Internet service providers, universities, government agencies, and large tech companies all typically run their own autonomous systems. Each system is assigned a unique identifier called an ASN, which other networks use to recognize it and exchange routing information with it.

What makes a network “autonomous” is that it controls its own routing decisions. The system decides how traffic moves within its borders and negotiates how to exchange traffic with neighboring networks through a process called ISP peering. The glue that holds all of these systems together is BGP, the protocol that autonomous systems use to advertise which IP addresses they’re responsible for and learn about routes to everywhere else.

When you send data across the internet, it typically passes through several autonomous systems before reaching its destination. A traceroute makes this visible, showing each network your traffic moves through along the way.