An Autonomous System (AS) is an independently operated network with a unique identifier number assigned by the RIR
- An Internet service provider, or other organization, that operates a network and wants to participate in interdomain routing
- Some organizations operate more than one AS
- For ease of administration
- Due to company mergers
Topology (IPv4)
AS-level topology – what are the inter-AS connections?
- Each node is an autonomous system
- Position around circle based on geographic location
- Distance from centre based on number of links to other networks → more links, closer to the centre
- Edges show links between autonomous systems
- A potential route traffic can flow
- Not all possible routes are in use
IPv6
The graph is much sparser, but IPv4 has had a 30yr head-start
Routing at the Edge
Devices at the edge tend to have simple routing tables
- The devices on the local network
- A default route for everything else
- e.g., routing table for hosts in Glasgow SoCS:
Network: Netmask: Gateway:
130.209.240.0 255.255.240.0 eth0
default 0.0.0.0 130.209.240.48