What is the difference between inter-AS and intra-AS routing?

The widespread belief that the Internet is a single entity is not entirely true. This is because while the Internet certainly "seems" to act like a single entity, it's a vast networkA network consists of a set of computers that are linked in order to share resources. comprised of many smaller networks.

Each of these smaller networks comprises routers connected through links, and these networks communicate by adhering to the Internet Protocol (IP)The Internet Protocol is a set of rules that are used to send packets over networks..

The illustration below depicts a network comprising two routers connected through a link, containing two nodes each.

Autonomous systems (AS)

The smaller networks referred to previously are called Autonomous systems (AS). They're called autonomous because each is independently operated and maintained by one or more network operators.

Every Autonomous System is assigned a unique autonomous system number (ASN) that identifies it. Along with the ASN, autonomous systems are also given a set of IP addresses with an identical prefix to identify their connected nodes. For example, Google has an ASN of $15169$ and is assigned $23,124,992$ IP addresses.

Since every AS is independently operated, Google's network administrators and operators are entirely responsible for anything regarding the AS, such as link failures, network attacks, maintenance, and many more.

Inter-AS routing

The prefix "inter" is synonymous with "between". It follows that inter-AS routing refers to routing that occurs between autonomous systems. As the Internet is a collection of autonomous systems, inter-AS routing is vital to making the forwarding of data packets from a source node to its destination a reality.

Inter-AS routing works only because the autonomous systems are assigned unique ASNs, and can share routing information across links that connect them. The routing information shared depends on the deployed algorithm, with the Border Gateway Protocol being the most eminent.

Note: To learn more about the Border Gateway Protocol, refer to this Answer.

The diagram below illustrates how inter-AS routing functions between different autonomous systems. If a node in the AS with ASN $871$ wants to send a packet to a node in the AS with ASN $100$, it will use inter-AS routing applied to the red links.

Intra-AS routing

The prefix "intra" is synonymous with "within". It follows that intra-AS routing refers to routing that occurs within a particular autonomous system. Again, since one or more network operators are in charge of a specific AS, they must ensure the operation and maintenance of the intra-AS routing.

Like inter-AS routing, the routing information shared during intra-AS routing depends on the routing algorithm used. The following algorithms are the most widespread:

• Distance Vector Routing (DVR)

• Link State Routing

The diagram below illustrates how intra-AS routing functions within an autonomous system. If node 2 wishes to send a packet to node 5 in the AS with ASN 100, it will use intra-AS routing applied to the blue links.